I don’t even… how do you prevent that?

The usual way?

What’s the usual way of stopping someone from collecting rain water?

Fines/violence, usually.

It’s actually not uncommon to have laws that restrict gathering rain water in many places. Lots of US states do as well. If water is collected locally on a mass scale, it messes with water tables/rivers/lakes/etc.

Forbidding it when a place doesnt have otherwise dependable water infastructure is inhuman however.