The version I was told at a concert was there was a risk of people re-capping the bottles (full of unmentionable liquid) and then throwing them into crowds. Though I expect “we can make more money” is more likely.
Regardless, it’s easily solved by sticking a spare cap in your pocket before you go through “security”.
For a couple of hours? Nah, it’s more easily solved by just not taking water into the venue.
It’s far more likely that you’ll get bottles of piss being lobbed around at a festival, because they’ve had hours to drink beer, and it’s too far to walk to the bogs, and walk back.
Or else they just piss down the back of your legs. Or Prodigy fans do, at any rate. In my experience, anyway.