For the record (and flame-avoidance), I tip what most Americans tip. That said, I don't follow your logic. In most places in the world where workers' wages fully compensate them for their work, there is no practice of tipping. In such places, a patron might leave a tip only if the service they provided went above and beyond the job they were paid to do through their wages.
The tipping practice in Cozumel seems to be an odd hybrid of US and other countries'. The US custom of tipping has become prevalent because so many of us visit the island, yet unlike dive staff in the US, Mexican dive staff are apparently fully paid by their employer--if I understood you correctly.
"tipping" is not only a US custom - and it is not a new thing here with regards to tipping dive crew.
They would NOT go hungry without tips, but regardless of what they are paid by their employer, they are in service positions where tipping is customary. Some are paid more by their employers than others of course - there are some shops that still pay really crappy salaries because they are still paying based on 20 years ago rates.
On a liveaboard, the crew is paid well - I still tip. Hair dressers and those kind of professions are paid well, tipping is still customary and I tip accordingly. I could increase this list of examples, but I think that makes the point