Each minor league player makes a choice as to whether the compensation and risk are worth it for him. They should be paid more from the standpoint of fairness, in my opinion, but people's opinions differ in terms of what they see as fair. Their compensation is what it is because the union doesn't care and the clubs have found that anyone who has enough talent is willing to play at these salaries (and bonus amounts depending on potential)--they are not losing many prospective major leaguers to Joe's Auto Sales or FedEx.
The union can fix this problem by taking a very small percentage of what the superstars make and redistributing it to minor league players. The clubs could fix it by redistributing profits to minor league salaries, but they have no incentive to do this since players are voluntarily willing to play at the low salaries. I'd like to see the minor leaguers make more, but I'm not willing to compel the union or the clubs to make that happen.
I honestly don't see that the minimum wage law applies to a situation like this--players receive benefits in lieu of salary--gloves, bats, uniforms, meals, cheap rooms, coaching, training, and whatever they estimate the benefit of possibly advancing to AA or above is (AA and above players get compensated pretty well).
People in many industries do this all the time by interning, often for free. Internships at financial houses like Morgan Stanley are very competitive and for a long time were not compensated at all--people did them hoping to land full-time jobs.
The low minors are a poorly-compensated internship. Two guys from my high school class were drafted, and both washed out early. Neither received a big bonus, but they both knew what they were risking. Both of them have very good jobs now (one in construction, one in casino security). Would they have liked double the salary when they were in their late teens/early 20s? Sure, but it would not have changed their decisions, nor would it have made much difference in their lifestyles.
I really don't see this as something to get exorcised about, and I would really not compare a minor league baseball player to a serf--that comparison really implies no understanding of how true serfs really lived. Minor league baseball players are interns, gambling a few of their years of service for the chance of a decent payday (in AA or AAA), a large return (at the MLB level), or a lifetime career in coaching/broadcasting/product representation.
JMHO, and, honestly, it's based not at all on envy. Just respect for the decisions these players have made, and their intelligence (and their family's intelligent advice).