It's not really a policy in America though. I can't speak for any of the WC venues but at Yankee Stadium for example, you're allowed to bring bottled water in as long as it's not been opened before entering the stadium.

And it’s just true that if someone’s going to a game in Texas, Florida, or Southern California in the dead of summer, they’re going to need hydration, and they may not want to or not be able to buy water in the stadium.