the M1 is the only visa
englishal - not sure that soundbite is totally correct - I remember working (as production crew) on a post 9/11 seminar with FAA, FBI, INS, Customs, etc (in the days before TSA came along to screw things up).
They stressed that if anybody is in the USA on ANY visa (not the waiver) for ANY purpose - they could carry out flight training as a secondary event provided that their PRIMARY reason for attendance in the USA was supported by any make, model or colo(u)r of visa.
I'm not saying that this is, necessarily, still the case - but it used to be.