I know this is an old thread, but several points have not been made in it.
1. Many of the practices described in this thread are violations of the law, as the first dive shop I worked for discovered before changing its policies. Dive shops get away with it because some people don't know better and because the ones that do know better have decided it is better to live with it than report it to the appropriate authorities. (I am one who never reported those abuses to authorities, even when I confirmed with an expert that they were illegal.)
2, In some cases you hear of and report on threads like this, people are lying. I used to do all my diving on a dive boat whose captain had a regular practice of imploring divers to tip the DMs because "they work for tips." I finally challenged the legality of that in private, and the regular spiel became something along the lines of "the DMs depend on tips for much of their income."
3. The definition of professional for diving differs in large part because professionals are required to carry professional liability insurance. They are required to carry that insurance because of the possibility they might be sued. For that to happen, payment does not have to be involved; you are a professional if you are acting as one. What is involved is an understanding on the part of the plaintiff that he or she could count on the professional to provide some level of service, usually including safety. There is a myth perpetrated in ScubaBoard threads that DMs have no responsibility to be a safety factor when leading dives. That is simply not true. DMs are not expected to pull off miracles, but they are expected to provide a reasonable level of safety; in one famous case, the DM was successfully sued when he refused to share air with an OOA diver who died. In another case, two DMs (and their agency) were successfully sued when they failed to notice that a diver was missing when they called the roll. In that last case, the DMs were not members of the boat crew but were instead unpaid members of the dive club that had chartered the boat. In the discussion threads mentioned above, dive operation owners regular jump in to say that they absolutely demand that their DMs provide a reasonable level of assistance for safety, but those posts are routinely ignored by the posters who say DMs have no responsibility for your safety.
In some cases, there is an understanding that a role needs to be performed by a professional, even if the person in that role is not literally a professional. In a case in the San Diego area about a decade ago, a DM on a boat jumped into the water to help a diver who was having trouble on the surface while trying to get back onto the boat. The DM helped the diver out of his BCD, evidently not realizing that the diver had a weight belt rather than integrated weights. The predictable happened. Although the DM had been certified years before, he was not carrying liability insurance, so he was technically not a professional while acting in a job that required a professional. The DM committed suicide, and the shop went out of business.
I have been warned that if I were to buddy with an insecure diver and say, "I'm an instructor; you can count on me to help with this dive," then I am a professional even though I am not getting paid, and I can be sued. If I do not make that sort of statement, I am most likely free from liability, even if I am a known instructor, although there have been some very troubling cases in that regard of late.
