Even if the line training TRTOs were required by law to pay the minimum applicable hourly rate all that would happen is that it would be front loaded onto the course cost anyway. A 30k 500hr training cost on an A320 would just go up to 34k. Actually probably a little bit more than that to cover for the "administration" expenses of putting the trainee onto the companies books.
And you can guess that this added cost will in no way deter the canon fodder.