I don't have a problem with a dive op charging me if I cancel within 24 hours of departure, and I would not have a problem with a dive op that had a minimum diver count to launch the boat. I do have a major problem with dive ops that do not notify you that the trip is in danger of being cancelled until you show up at the dock.
I don't really have a problem with it either- their business, their boat, their policy. I'm not going to take it personally but I'll protect my dive trips in every way possible- and that includes a) scheduling with Ops that have a low or no minimum policy or b) book dives with several dive Ops, and then calling a couple of days before, checking who has got enough divers and cancel the rest. Doesn't do them any good, that's for sure, but THEY started it.