I'll agree that clubs running "IDPA" matches should be affiliated. Everyone running a match should want to do it right, and affiliation, along with certified SO's make for a better match.
To answer the question: I think a lot of IDPA CoFs become IPSC'd, and it's mostly because of the reality of setting up CoFs on square ranges, and the desire to shoot. It is easier to set up CoFs that have downrange movement, and it is more fun to shoot 20 rounds than it is to shoot 10. One of the things that we do at our club matches, to help retain the "defensive" nature of course designs, is to all but forbid downrange movement. Obviously, if there is cover directly between the shooter and threat targets, the shooter goes to cover, but we don't do any "house clearing" where the shooter goes deeper and deeper downrange, "hunting" for threats, without sufficient rational justification in the scenario. We start a lot of stages with T1 at contact distance, and the shooter backing to cover at the buzzer; emphasizing that the shooter is trying to get away from threats, and not going looking for them. Ken Hackathorn, in a recent Tactical Journal, suggests upping the number of scored hits per target, rather than adding targets, to get the round count up. I like scenario CoFs to have no more than four or five targets, max, so three rounds per gets you a good round count.
One of the problems that seems to be associated with IDPA "like" matches, is that it's difficult to enforce the rules. Who's in authority? If nobody really knows the rules, it's easier to ignore them. If nobody has the nerve to enforce equipment rules personally, just post them before the next match, and tell everyone that they'll have to conform if they want to shoot. The problem with this, is some people won't want to comply, and maybe won't come back. If that's a concern, that you'll scare away your "customers" by enforcing the rules (I've heard that one before), then you will still have to come to some kind of consensus about what everyone can live with. I think that's tougher than enforcing rules that are already widely accepted.