

Users get a service, so it can be argued they are paid in kind. That’s the price of their “free” services.
Whether you agree with that or not, websites are unlikely to pay users to use their services (unless they’re at least providing content) any more than a coffee shop would pay its customers to drink their coffee.
Big Clive (electronics channel on YouTube) took a look at this yesterday. Analysing photos of the event, he determined that it was likely the lighting by the toilet facilities (or rather what looked like a sort of toilet-themed art installation, presumably near the facilities) that may be responsible, as they appear to have used clear unfiltered UVC lamps to light the area.
It could be that a sub-contractor might be the culprits rather than the organisers themselves (although ultimate responsibility may fall with them).