So for those student-athletes they will be going "pro" in something other than athletics. Plus, just because someone ends up playing professional sports doesn't necessarily mean that was their plan all along. For some the plan is to go to college and get your undergraduate and sometimes even your graduate degree. If you happen to get the opportunity to play professionally, then great. If not, then you and/or your family won't lose any sleep over it.

Besides there may not be a semi-pro league for every professional sport out there. If a professional league doesn't deem HS graduates eligible like MLB does, then there may not be no other avenue for someone to hone their craft until they become eligible. I am not suggesting it is the responsibility of colleges to babysit these athletes until they are eligible. It just happens to be the world we live in now.

As far as intercollegiate competition goes, school do compete in them on the academic side (band competitions, acapella competitions, choir competions, debate competitions, engineering competitions, etc). Yet, they aren't governed by NCAA rules. Each competition may have their own governing bodies or at least rules to follow. I suspect those rules don't prohibit the college student from earning money outside of school during "competition season."