I don't recall ever having heard the word "charities" used as a synonym for "501(c)(3) groups". It hadn't occurred to me that anybody would think we're fund-raising charities. And if they do, I'd have guessed that they'd gotten hold of the wrong end of the stick by encountering discussions of charitable disposal of surplus funds and assuming that that's what conventions are for.

Additionally complicating the matter is that there are some cons which are charities in this sense. Potlatch, for instance, exists financially for the purpose of raising funds for Clarion West, though this is done specifically by the auction. However, each Potlatch picks a community charitable organization to donate surplus funds to, and this choice is announced before the con.