It's a difficult problem, prismite. People still want to play their characters as they are. If you're offered a mission that your character would normally not take, being forced to take it because otherwise you're "being a dick" isn't fun. In fact, one could argue that, at that point, it's the GM that violated the social contract by offering the mission in the first place.
I do agree that, in general, people should just agree to the mission for the sake of the game. But that does introduce some oddities that need to be addressed. We had a discussion about that in-group a while back. Is it fair, for example, to penalize players for choosing to run a mission (for example, with loyalty reductions or notoriety)? After all, the characters choose to take the run. Except that the players didn't get a choice in the matter, since, in practice, they're forced to go along with the run - so penalizing them for it is unfair.
In the end it's certainly not as simple as always having to go along with the GM, or conversely, of always being free to decline a run.