Up until the year 2012 collecting rainwater in barrels and tanks was illegal in California because it diverted the rainwater from the ground where the cities could collect the rainwater and sell it to the people.
It was mainly due to drought and greed that rainwater collection became illegal in California and some other states.
Rainwater is a natural resource that should not be made illegal to collect because no one owns the rainwater unlike some some places think they do.
It's basically that California and some other states believe that they own the rainwater that comes down from the sky because the cities want to profit off you for the rainwater that should in fact be free of charge.
Now it's no longer illegal to collect rainwater in California for personal use so if you're in California and want to collect rainwater you can do so.