In Maui, that's always the case. I think I've read it's a regulation there. Sometimes there's 2.
In Hawaii there is wording that is open to different possible interpretations. I don't believe anyone has asked for a definitive interpretation, for fear that the most restrictive one will become "law." I can not find it online now, but I have seen it before. The wording is something like "divers must be under supervision."
A more plausable theory is that many dive charters here have had many experienced divers make negative comments with regard to Hawaii diving being boring and barren. Compared to other dive destinations, the unique nature of Hawaii diving does not hit you in the forehead like a ton of bricks.
We have a higher percentage of endemic species than most (any), but the mostly hard coral landscape means you must search for the rare stuff and know what it looks like. Now even the bottom end of scuba charters go out of their way to point out the stuff we have here that you can't see anywhere else, as well as typical stuff that hides very well.
By paying for the extra expense of a instructor/guide, vs the lowly DM of other locations, less of the experienced divers bad mouth the diving in Hawaii. Photographers are especially visible now, with online photo posting. The guide that shows the photog many good shots garners many more word of mouth customers. The reason there is very little dive taxi service here is probably due those businesses going out of business. In order to survive, we give you a guide.
Those crotchety old dive taxi fans are also notoriously cheap, and probably complain about the price of resorts and restaurants as well. Better that they stay in a condo after stopping at Costco and only do shore dives. At least then they only complain about the boring diving.
As usual, I am exaggerating a little, and there are quite a few SB members who are condo/Costco/shore divers AND do like Hawaii diving. The above ramblings are typecast generalizations and only apply to 95% of the tourist diving in Hawaii.