Last time I was in the USA (Around 1 month ago) at a flight school which is approved under SEVIS, I bumped into a Senior Immigration official. Before I get on to what he said I might as well share his appearance:
He wore a cowboy hat, had white hair and moustache, and wore an outfit resembling a cowboy from the west - which kept me pretty entertained as an interesting image on immigration.
When asked about this he said the same kind of thing as :
Do not qualify is not the same as "do not need one".
To cut a long story short he explained that anyone undertaking a course of flying would be doing more than 18 hours of study so would need the M-1 visa.
The schools which trained people on B1 visas are lying or do not understand the implications - as stated before YOU are at risk!
This happened to a school recently in Washington he mentioned to me.
They did training without M1 visas (he did not specify what status the students had) - the school was closed - the students expelled from the United States with the 5 year ban and the owners and instructors (who apparently have a responsibility as well) have all be subpoenad.
Use that along with the tailwinds flight school story and that should warn most people off.
As far as he was concerned there is no provision under US immigration laws for flight training to be conducted by a foreign national at a non SEVIS school.
Interesting stuff.