Probably the main reason, is to keep said stuff from getting too thick in a given area. While a bit here, a bit there is not a big deal, when you put those bits on top of ea other in one spot, you then end up with places that fall into the toxic waste problem. Then in some cases, one needs to spend money to contain, or clean up.
This is one of those, I use chemicals at work, ie insecticides, herbicides etc, but prefer to keep the use to a minimum. I use them, but unlike some folks, that avaid them like the plague, one has to have trade offs. But, if you rub cleaned you boat bi weekly over a period of time, granted in salt water, the tide would move some of said compound around, but MOST, would end up under your boat, creating its own little pile of copper or equal that would/kill organisms under you. This is to a degree what the marinas are trying to avoid, such that they get fined etc
Your boat itself is not the issue, it is when your boat plus a dozen others do the same thing ea day that eventually causes issues.
Same as DDT yrs ago, a little here, a little there, and all of a sudden, we have the US national symbol on the brink of extinction because the DDT went thru the food chain, from insects to small animals that bald eagles eat, and they are not laying eggs with thick enough shells, babys are not living etc... So any way, off on a soap box, but with similar issues. Same as locally Dursban, a good insecticide for bugs, that has worked thru the food chain hurting local salmonoid runs. SOme of what I said is not the only issue, but part of the overall issue with things.
Cut them ie owners of the marinas some slack and move on, not the end of the world......yet.
She drives me boat,
I drives me dinghy!