The Guitar is one of the most powerful weapons in the game, allowing you to turn Conservatives into Liberals. Conservatives in the game often say when they die "Better dead than Liberal..." Conservatives in this game would rather die than turn Liberal, and using a Guitar, which can turn them Liberal quite fast if you have high Music skill, puts them at risk of what they consider a fate worse than death, namely becoming a Liberal. So naturally, to avoid a fate worse than death, they are willing to use deadly force.
It's not as if the Liberals in the Liberal Crime Squad are any different though. If a Radio Personality, for instance, tries to convince them to become a Conservative, by reasoning with them using Good Old-Fashioned Conservative Wisdom, the unhinged Liberals in your Squad are liable to counterattack the entirely nonviolent Radio Personality using deadly force. In this game, anyone views a threat to their ideology as a threat to their life.
Playing a Guitar ISN'T a crime though. And Loitering is what you get charged with if you didn't commit any actual crimes, it's just there so that if you get arrested for no reason other than pissing off Conservative Police Officers, they'll have SOMETHING to put you on trial for. But the penalty for Loitering, even if you are convicted, is nothing... except maybe if the Death Penalty is C+, since I think when the Death Penalty goes to C+, EVERYTHING becomes punishable by death.
And playing a Guitar isn't considered assault, either. It DOES alarm Conservatives, though. But it doesn't alienate Moderates or fellow Liberals. Moderates and fellow Liberals see nothing wrong with playing a Guitar, only Conservatives take offense to it.