People have a right to be frustrated with teammates (usually townies) if they give what they feel to be a sub-par performance, either effectively undoing all their good work earlier in the game, or because they fail to see what they deem to be obvious scumtells/powertells. I agree though that leaving it at that and expecting 'culprits' to learn from their own mistakes is an unproductive course of action. Chris has tried to improve peoples play in the past, whether tongue in cheek or not, but the truth is there's only so much you can teach people. You can tell people how to play and what to look for, but ultimately some people just lack the ability to rise to the occasion and/or act on what they've learned. Furthermore, a surprising number of players just fundamentally don't have common sense or the ability to rationally think things through, which is basically the one trait in common with all the best players (usually).
You're right in that a good place to start would be encouraging people to be more assertive, and not so easily lead by the mafia, whilst not going so far as to completely ignore, or even distrust the opinions of your teammates. It can make you sick to the stomach seeing how many games towns throw away in end-games because they let that final scum wriggle their way out of a hole and explain away all their scummy behaviour earlier in the game.
--
"Eet's game time..."