Appropriate places such as?
I'm not saying that expressing hate at Bush is the best outlet, and i can't deny that some of these musicians MAY be inclined to "follow pop culture". But like you said, by playing music in front of "impressionable teens or just music fans who don't care for politics" it's really up to the fans to accept the message being delivered or not. If they're impressionable, the lyrics would have done its job and these people (armed with newfound interest in politics) may want to go out on their own to learn more and hopefully be more aware of the present political climate.
Likewise, if they're just in it for the music, then good for them. Being such an accesible medium, mixing both music and politics can't be avoided at times and it's up to the listener whether they're willing to be impressionable or not.
Regardless, i understand what you mean when you say that some people hate Bush just to go with a trend -coughAmericanIdiotcough-. Musician and fan's prerogative, i guess.
On another note, there're people out there who you can't help but just hate.