Would you seriously consider a driving school where they put you on a fenced-off traffic circuit with only small size sedans driving around, no trucks, no pick-ups, no RVs, no bicycles, no motorcycles?
Well, they actually do that. I remember my drivers training setup started with some weird video-tape simulators, and moved to a private parking lot with cones. Removing complicated variables during initial training really isn't new or uncommon. It's really just reducing task-loading.
Of course, I know your point is that eventually you have to deal with these variables. Sure, as these students dive more, they'll see all sorts of truck and moped-style BCs, but there's no reason why their first confined-water dives need to be with current, pinnacles, sharks, and an overhead environment (so to speak). It's just easier to teach a group the basics when you don't have to say "OK, to inflate your BCs, you hit this button, you hit this other one, yours has it routed to the right side, yours is actually this lever on the BC itself, and yours doesn't have an inflater." After all, once they know the basics, it's pretty easy to see what does what, whether you have a jacket or a BP/W.
As for the DM/shopowner debate, as others have noticed it really just seems an inevitable conflict when shops offer instruction as a gateway to equipment sales. It's really hard to find absolute fault with either position.