I would still like to see where it is specifically written that you need an M1 visa for a flight training course...
I used to regularly do "training courses" in Houston and admitted under a B1 (business) visa, not a student visa. For an already qualified pilot, training for a rating is effectively a "training course" and you are not a student. It doesn't even have to be full time, and I think that the M1 is for a "full time non academic course". What if you want to go for a months holiday, and do a bit of flight training here and there and end up with a ME rating?
I can understand an M1 for a "student pilot" on an FAA student permit, or maybe someone who wants to stay in the US for a number of years earning money. You can be employed as long as it is to do with your course......which is how a lot of instructors get away with it, they are "training" to become airline pilots.
I stand to be corrected but would also like to see clarification....
Cheers