In any case, while this was apparently a pretty "lame" party, it appears to have been an important part of the culture of the firm, and OP didn't jibe with it very well.
There was thanksgiving food; there was cake; a few people brought small children; my boss played an instrument and sang for us. It was great.
Basically I agree with you that the "party" described was lame. The problem isn't that there was beer at an on-premises party even though OP doesn't drink. The problem is (definitely) tacit mandatoriness and (to a much lesser extent) that the party didn't have anything to offer other than a chance to show that you're "dedicated".