Maybe someone can enlighten me.

Sure nothing to do with any of those 4 reasons. But they probably figured it would generate political 'debate'. In that case, they should add that to the list of reasons.

@secretguy definition of spam…

irrelevant or unsolicited messages sent over the internet, typically to a large number of users, for the purposes of advertising, phishing, spreading malware, etc.

In an audio forum this message you posted

Congrats and I feel ya. The current state of the country has stolen most of my joy.
 

is irrelevant and unsolicited and is sent over internet to a large number of users, advertising your political views. 
 

It’s hard to argue with the action taken by the moderator. I get it you don’t like it. If you feel like expressing your political views you can do so on public property as a protest. If you do it on private property, which is conceptually similar to doing it on this forum, you will be removed. And it could hurt more than just emotionally. 

@audphile1 +1. Thanks for the clarity.

In this case if the OP is seeking “Justification”, it’s a poor goal:

1. What is the purpose of opening this thread? To blow off steam? To line up on your side? To try to embarrass/pressure the Agon moderator to get your way - an explanation? We do not know the “Justification”.  
2. Agon makes the rules, we are not “entitled” to an explanation 
3. The Agon moderator has little incentive to explain their actions - more work and possibly increased grief

It’s best simply tone it down and move on.

+2 @audphile1 Nicely put.

Tom