To undertake training - you have to be 'sponsored' by the Flight School
To be sponsored the Flight School has to be SEVIS
Only 141 ops only though.
Catch 22 - you can only get an M1 visa from a flight school which has SEVIS and the only ones that do are 141 schools.
So it is a nice little earner for the 141 schools - "you need an M1 therefore you must go to a 141 school because they are the only ones that have SEVIS" - which is why they keep going on about it.
However, if like me, you do/did all your training under pt 61 (i.e. freelance instructor), who don't have SEVIS and therefore can't sponsor you for an M1 then I assume that alternative immigration status is ok, because you are not doing a course of study which requires an M1....
Just my interpretation of course