Its all a bit of a grey area. If you read about the M1 visa, it is designed for a student, who wishes to study a non-academic course.
It could be argued that if you are already a pilot, holding a pilot certificate you are no longer a student pilot (in the eyes of the FAA) and therefore not entitled to and not needing an M1 visa. I would like to see some clarification from the US government on this as I believe that there are tons of people wrongly under the impression that they need this visa, when in actual fact they don't. I also believe that flying schools are wrongly applying for visas for foreign pilots probably because they can and they know it works.
The US Embassy initially refused a friend of mine an M1 for his CPL in FLA (but subsequently issued it) becasue in their eyes he was not a student and did not need one as he was already a pilot.
I hope with this TSA stuff coming out then this will clarify the matter and get rid of the need to have an M1 for additional ratings.