Well, the only response I have is that either you are exceedingly unlucky or you are somehow being flagged as a target. I have a lot of data that runs counter to what you are attesting to; I have dealt with the Cozumel police a few times and they have always been courteous and respectful to me, as I always am to any policemen I encounter.I've only been to Coz 20 times in 15 years, and I've been stopped by police every time I rent a car and leave San Miguel.
The last time, the police came up behind and started passing me, then then he saw that I was a gringo, dropped behind me, and turned on his lights.