Your question and comments don't apply only to maple. They are equally relevant to any type of wood. A search of the archives will turn up some threads on glass versus metal versus wood racks/stands.
Why is maple often used as opposed to any other wood? Well, it's hard with good stiffness and rigidity. A lot of baseball bats are made with maple because of this. These characteristics are augmented by the fact that it does not have a lot of knots. It's dense, as you note. The grain is very tight. Its grain doesn't split with time like oak has a tendency to do. Many woods have these characteristics but some of them are exotic or otherwise expensive because of limited availability. Maple is common, readily available, and inexpensive. It's kind of bland and neutral (except for curly or bird's eye maple), even when stained, but it fits in with most decors because of this. That's why maple is one of the standard finishes for many speakers, along with cherry or basic black. It has good WAF, especially when compared to many metal racks that often have a macho, industrial look to them.
So for all of these things, it's very practical if you want wood rather than glass, metal or veneered MDF.
Now, what about the acoustical properties?
All the things you say about ringing and vibrations are of course correct. However, it applies to everything else too, including glass or metal stands/racks. All physical substances have a resonant frequency and will "ring" if you hit the right frequency. The platter on my Linn turntable rings like a church bell. So you deal with it the same way you deal with any other type of rack you have, or with any other component in your system. You alter the resonant frequency in the way you construct it to make it less audible; you isolate vibrations; you drain vibrations away, etc., etc. In your post, you seem to anticipate and dismiss the draining away of vibrations as a satisfactory answer. However, that is one of the techniques used. So like it or not, that is one of the answers. And you can find many, many threads in the archives on vibration control, both in theory and in practice. I cant possibly repeat all the points made.
Your point about the component not being isolated from the stand/rack is a bit difficult. Many techniques of vibration control, and products made to assist in this, do isolate the component from the stand/rack. And many components themselves are constructed with internal vibration control or isolation.
I cant resist making a few comments about your posts points about electric guitars and sustaining notes. Ive made a couple of solid body electric guitars as a DIY hobby. It was part of a guitar making course I took at a local pro shop. Im not a pro or an expert, but the guys who taught me are. The choice of wood for a solid body electric guitar, as opposed to an acoustic guitar, isnt typically related to its acoustical properties. It is certainly possible that some musician may come into the shop to have a guitar made and say such a thing. But its a bit of a stretch. Any acoustical properties of the wood in the guitar will be overwhelmed by the tone control/settings on the guitar amp, or any effects pedals/boxes. The guys at the shop told lots of stories of pro musicians who came in to have their guitars made/modified a certain way for whatever reason. Well, the customer is always right so they do what theyre asked to do.
Anyway, to succinctly answer your initial question, the theory behind maple racks/stands is the same theory that is behind racks/stands made out of anything else, wood or otherwise.
Why is maple often used as opposed to any other wood? Well, it's hard with good stiffness and rigidity. A lot of baseball bats are made with maple because of this. These characteristics are augmented by the fact that it does not have a lot of knots. It's dense, as you note. The grain is very tight. Its grain doesn't split with time like oak has a tendency to do. Many woods have these characteristics but some of them are exotic or otherwise expensive because of limited availability. Maple is common, readily available, and inexpensive. It's kind of bland and neutral (except for curly or bird's eye maple), even when stained, but it fits in with most decors because of this. That's why maple is one of the standard finishes for many speakers, along with cherry or basic black. It has good WAF, especially when compared to many metal racks that often have a macho, industrial look to them.
So for all of these things, it's very practical if you want wood rather than glass, metal or veneered MDF.
Now, what about the acoustical properties?
All the things you say about ringing and vibrations are of course correct. However, it applies to everything else too, including glass or metal stands/racks. All physical substances have a resonant frequency and will "ring" if you hit the right frequency. The platter on my Linn turntable rings like a church bell. So you deal with it the same way you deal with any other type of rack you have, or with any other component in your system. You alter the resonant frequency in the way you construct it to make it less audible; you isolate vibrations; you drain vibrations away, etc., etc. In your post, you seem to anticipate and dismiss the draining away of vibrations as a satisfactory answer. However, that is one of the techniques used. So like it or not, that is one of the answers. And you can find many, many threads in the archives on vibration control, both in theory and in practice. I cant possibly repeat all the points made.
Your point about the component not being isolated from the stand/rack is a bit difficult. Many techniques of vibration control, and products made to assist in this, do isolate the component from the stand/rack. And many components themselves are constructed with internal vibration control or isolation.
I cant resist making a few comments about your posts points about electric guitars and sustaining notes. Ive made a couple of solid body electric guitars as a DIY hobby. It was part of a guitar making course I took at a local pro shop. Im not a pro or an expert, but the guys who taught me are. The choice of wood for a solid body electric guitar, as opposed to an acoustic guitar, isnt typically related to its acoustical properties. It is certainly possible that some musician may come into the shop to have a guitar made and say such a thing. But its a bit of a stretch. Any acoustical properties of the wood in the guitar will be overwhelmed by the tone control/settings on the guitar amp, or any effects pedals/boxes. The guys at the shop told lots of stories of pro musicians who came in to have their guitars made/modified a certain way for whatever reason. Well, the customer is always right so they do what theyre asked to do.
Anyway, to succinctly answer your initial question, the theory behind maple racks/stands is the same theory that is behind racks/stands made out of anything else, wood or otherwise.