They get paid a reasonable salary to be trained in a job that will provide them a nice income for the rest of their life. Having a lower salary while in training is not unheard of in any field, I am not sure why you believe those numbers are unacceptable.
Further Baltimore shows why a national $15 min wage is untenable, as all that does is make more jobs "minimum wage jobs" because taking the min wage from $8 to $15 does not magically mean all the jobs that paid $15 now pay $22, that is not how economics work