It's a rule that was put in place in our marina about three or four years ago. The reason is environmental. Wiping down a bottom with ablative paint on it puts large clouds of "stuff" in the water, some of which is the copper in the paint. So from a strictly environmental point of view the restriction is probably a good idea. So far as I know these rules are set by the individual marina operators--- I don't think there is any sort of state requirement (yet).
Apparently some marinas up here differentiate between ablative and hard bottom paints. Ours does not.
Eelgrass, which grows in waters less then about 30 feet deep around here, is a critical element to marine life in the PNW, sheltering fish and providing a breeding ground for the little shrimp and other critters that provide food for everything from Dungeness crabs to young salmon. Given the damage done to the native salmon and steelhead populations for a whole host of reasons, there is much more awareness in the region now of what affects marine life. One result has been a number of new regulations aimed at protecting existing eelgrass beds and promote the formation of new ones.
Pollutants like copper and other heavy metals getting into marina waters as the result of cleaning activities is seen as another threat to the life in nearby shallow marine zones.
Divers in our marina can change zincs, do underwater repairs on hardware, and remove barnacles from props, shafts, and through-hulls. But that's it. I don't notice any growth on our bottom in the normal 12 to 18 months we go between haul-outs. We use Petit Ultima SR on our bottom, which has a very high copper content. At every haul-out the bottom is basically clean other than whatever barnacles have managed to attach themselves to the prop hubs, shafts, and rudders. We have a large, full-service yard attached to our marina (Seaview North) so hauling out for us is matter of untying the lines and idling about 200 yards into the Travelift so it's not an inconvenience.
Mpre than you wanted to know probably, but that's the deal up here. Given California's general attitude toward environmental issues like this I'm surprised this restriction hasn't been put in force down there, too.