Why was Mario imprisoned, and then forced to clean up Delfino Island, when there was absolutely no proof that he was the perpetrator?

The only "evidence" they have that it's him is a shadowy figure painting M's around the island, but even then Mario has an alibi, he was with Peach the entire time the event occurred.

Also, why was the island kept dirty the entire time, why didn't Delfino's government attempt to clean any of the paint up?