There's nothing wrong with MTN's policy, just the lack of enforcement, and the WASP deserves what's coming to them. Bear in mind it was the WASP's client that was doing the bad stuff. The WASP should have cut them off as soon as it was brought to their attention, and it shouldn't be up to MTN to try and isolate the bad service's traffic from all the WASP's traffic.
One of the things my business does are the same as this WASP. Now and then we pick up a customer doing dodgy things, and get the WASPA notice, but do quick investigations and cut them off if necessary. If MTN shut down all our binds every time WASPA caught one of our clients doing something, that would be disastrous!
Things aren't always so black and white. I hope the WASP is suitably punished (their clients have to go somewhere - more for me, yay) and all the affected users should have their money refunded - the MNO's keep this money back for 30+ days in case of this kind of problem, so it's not like the bad client can take the money and run.