Dgordon that's because of interference from the transformer. If you ever open up the power supply of your SLP-05 (as I have done many times, to replace those light bulbs that keep blowing!) you will see why it is safe to stack them.
The other quoted reason is mechanical grounding of the component base. I'm not a big believer of cones or support bases even though I have them. The only reason I don't believe them is because they don't work for me.