I guess your options are: boo, write a letter, boycott the Brewers by not buying tickets/merchandise, heckle players/management at the game or in public if you encounter them, etc. You are obviously "allowed" to do any of that - nobody is arguing that you're not. If you think that any of that makes a difference or makes you feel better, then by all means do it. Booing your own players does not make me feel better and I do not personally believe it makes a positive impact on the team, so I choose not to boo. Who knows though, maybe I should.