This is a great point, and one I haven't seen anyone else bring up. I'm a little irked this hasn't crossed my mind either. But it's true that the NFLPA is going to pressure players to get all they can because its entire existence is based on improving things for the player. Accordingly, the NFLPA would frown heavily on players making deals contrary to that agenda. Obviously when a player reaches Tom Brady status he isn't concerned about backlash from the NFLPA, but a young player getting his first big contract is a different situation.
Speculation here but a distinct possibility.
The whole point of a union is to ensure the players are not taken advantage of and that they can point to the fact the union ensures high dollar contracts for everybody.
That's what the main reasons are for a union.
There are other reasons like pensions, medical benefits and political clout.
But the union can not be caught insisting that all players adhere to their rules..
while on the other hand allow a personality like Brady artificially depressing the market by taking less along with an eye wink.
Very risky. Federal Anti-trust laws are at risk there.