Also it's a private event/venue, not a public function secured by public officials (police). Anybody can ask you to do almost anything to grant you access into their private property, as long as you also have the choice to just go away. In the same way that private security guards have only a subset of the powers of the actual police, they are also required to follow only a subset of the same procedures. This is all perfectly fine, because if I was hosting some kind of event where I had a vested interest for some reason in people not bringing in their own alcohol, I'd expect to have the power to make sure they didn't.