The thing that is missed is that no one is forcing a player to play in college. There is a free market and high school seniors are welcome to explore that market to see what their skills will bring. Once they do that analysis, in football, almost 100% of them realize the best, most lucrative offer they have is to accept the college scholarship, as the NFL will not consider employing them for 3 years.
This is the rub, if there was a minor league system set up to pay the high school senior an equivalent (or greater) wage than the scholarship provides there would be an argument that college players are not paid their fair market value. This is not the case, in fact the case could be made that the college players are being over compensated with a $50,000 a year scholarship as the open market in football would pay them $0 a year and working a minimum wage job (as they only have a high school degree) would only pay about $30,000 a year.
The real issue is once eligible for the NFL are the players being underpaid. You could say yes but at that point the player has chosen, willfully, to keep playing college football. No one is forcing them or suppressing their wages. No different than after working in a job for 3 years and having another company offer a big raise you have a choice to jump ship or stay and hopefully get a better offer a year or two down the road.