My opinion about the "buy you a drink" thing is they do that instead of putting the money in the tip jar because there is a social expectation of "owing" something in return to the person who buys the drink--usually attention from the stage/performer or the expectation that you will perform what they want you to perform "just for them."

I find this to be very manipulative, and have no problem not touching or acknowledging the drink, whereas I always thank a person for putting money in the tip jar.