I don't think things are quite as restricted as you make them out to be, but pretending for a moment I agreed. . .

This isn't necessarily a bad thing. It's certainly realistic that the various power groups in the land would polarize things a bit and draw people to their philosophy that might not otherwise be inclined to take it as seriously.

You can see something like this in modern America with our political party system. I know a guy getting into politics who is some flavor of socialist. He basically believes modern Cuba is a paradise, Fidel Castro is a genius, and the U.S. should follow his lead. I can't recall his official party affiliation, but I assure you it's one of the two major political parties and not, say, the American Communist Party. He says it's basically political suicide if you want to be a Senator or whatever to belong to anything but those two parties, and he's probably right.

Given that, is it that illogical that any random neutral mage (or whatever) might find him/herself drawn to Nexus/Sylvan/Tribunal/whatever philosophy and find themselves trying to take it seriously?