Star college players rarely stay at their schools for the full four years, but should it be mandatory in order to enter a professional draft?

In the NBA, players can enter the draft after only one year of playing in college (provided they are at least 19 years old). In the NFL, they don't require a full four years, but a player must be out of high school for three years before to reach eligibility. The official rules for Major League Baseball are a bit confusing, but teams can draft players right out of high school.

With many breakout college stars from the NCAA Tournament declaring themselves draft-eligible for the NBA, obviously some are leaving before they obtain a degree. Should players have to go all four years and graduate before entering professional sports?