Will it stop some individuals from coming to the US under Visa waiver etc and trying to get Flight Training - probably not. If they got caught and kicked out - would they tell folks on PPRuNe - probably not..
I have a visa - a B1OCS but not an M1.
If travelling to the USA for a primary reason that is not flight training - i.e. a business meeting, then I can undertake flight training as I have entered the country on a visa and are doing vocational training of less than 18 hrs per week.....When I did the seaplane rating I never got an M1, because I was there for another reason and decided to add the SSEA rating as a recreational course "for fun" of 6 hrs.....no M1 required.
US pilot factories "play it safe", and they don't know the different classes of visa any more than the INS
and so they automatically say "M1"....But the difference between an M1 and say B1/B2 is chalk and cheese - a B1/B2 is multi entry and valid for 10 years and is easy to get.
Of course if your course is a long course (0 to ATP for example) then an M1 is a better visa to get as it allows one to earn money whilst doing "training" for your course of study - i.e. a Flight Instructor on a "0 to ATP course" can earn money as a FI /Fly King Airs to support the remainder of his course (i.e. the ATP).....
Actually when a visa waiver is stamped, you are either stamped in B1 or B2 anyway - business or pleasure - no difference from a visa at all. So in effect you have entered the country on a B1/B2 and can still visit those business meetings, or lay on the beach even though there is no visa.