That argument could easily go the other way as well. Why should the safety of the newer diver be jeopardized by what the more experienced group wants? Shouldn't the dive site chosen be based on the ability level of the least experienced diver? If someone on a mixed ability level boat wants a guaranteed wall dive then maybe they should charter a private boat.
While apparently there are many new divers who come to Cozumel for easy dives, notwithstanding the fact that Cozumel is better known for deep walls and currents, I believe that many experienced divers come to Cozumel for the deep walls and currents.
If you have a group of 8 divers and one diver insists on going to Columbia Shallows twice a day all week because they think it's dangerous to dive below 30', I can imagine the other 7 would get a little upset at some point because that's not what they believed they were paying for.
It makes no sense to hold back the divers that go to Cozumel for typical Cozumel diving because a newbie found himself or herself in the wrong place at the wrong time. Speaking from a business perspective, a dive op that continually annoys seven divers to make one diver happy will lose seven customers and only gain one in return. That's just not good business, especially in a competitive environment like Cozumel.
That said, there are some "advanced" sites that I believe should require a group consensus because they are a bit beyond what the average Cozumel site entails. But divers that put themselves on dive boats unable to safely dive the average Cozumel sites should either find a different destination where the average sites are much better for beginners, sit the "unsafe" dives out, or utilize the services of a private DM or even a private boat charter until they are up to diving the average sites.
How exactly where the expectations to high? The original poster got at least three different confirmations from the DiveOP that they would be doing a shallow dive.
There have been plenty of times when the McDonalds drive-thru voice confirmed my order, yet what I ended up with was different than what I ordered. At a small restaurant, it's easy to send my faulty order back and request that the chef makes me what I wanted (and hopefully not spit in it). But at McDonalds, I'd have to park, go inside, yell at the manager, and still probably end up with an incorrect order.
Smaller operations, whether they are restaurants, hotels, or dive ops, are usually able to deliver more personal service. When I dive with LU, for instance, and I tell the guy who answers my e-mail that I want something in particular, I'm pretty sure I'll get it. If not, the guy who answered the e-mail is also the guy leading my dives and the owner of the business, so I can be pretty sure he'll remember my original request when I remind him. (Not that I ever request anything besides nitrox, but that's only because I'm a very laid-back undemanding sort of diver.) On the other hand, when you use a big corporate dive op that franchises through big hotels, the DM on the boat is probably getting instructions from the dispatcher who is getting instructions from the dive scheduler who is getting instructions from the e-mail answerer. Too many cooks and any special orders are easily forgotten. You didn't want special sauce on your Big Mac, go back in line!