One concert I went to that didn't let you bring water in ended up getting shut down because the only water available was bathroom sink water or tap water in a plastic cup for $8. And they didn't let you bring your plastic cups into the bathroom to refill them.
Probably about a third of the people there had to leave from passing out/dehydration and a bunch left in ambulances. And this was a concert where probably 90% of the people there were on drugs, which in my eyes makes the whole water situation even more irresponsible.
So legal, yeah probably, but also entirely fucked.
I feel for people putting on events like this. Ticket sales used to get you halfway to covering costs, but that's ok, you've got bar sales to make up the shortfall.
If everyone is taking drugs though, you either double the ticket price, charge for water, or not put on the gig.
Sorry clarification, I can't being my metal water bottle in.
I don't think I've ever been stopped trying to bring in a plastic bottle, but that needs to be empty so technically not allowed to bring in water. My main point was just that they sell it where there is a captured market.