This is partly myth - it truly depends on the dive shop. Some shops use freelancers so that they don't have to pay all of the benefits or employment taxes that go along with full time employees. Some shops pay the minimum they can get away with which is not a livable wage. Some, the more expensive operations pay their crews wages they can actually live on - something to think about when price shopping for dive shops. My crew is paid very well, in fact may be one of the highest paid crews on the island - they are worth it! Payroll and fuel are our biggest expenses.
However, that does not in any way shape or form mean that they don't work hard to earn their tips. I encourage people to take care of their crew appropriately regardless of how much you think they are paid by their employer - their salary should not factor into the equation of how much you tip them.