It is hard to compare a regular student working to earn money and an athlete. Don't forget that the athlete probably puts in an additional 4-6 hours a day in order to play their sport for the university. Between practice and training, travel and game time. It is very hard to hold down a job, be an athlete and a student. They do work. They work as an athlete and pretty much as an entertainer, as they entertain large crowds of people on a regular basis. They contribute to large amounts of money being made by said universities. I like the idea that at the very least they can sell merchandise in their name. I think you need to give them some extra pay for some very basics that are not covered, and for the ability to have the same fun that other students have. You are suppose to have the same student experience, but if you can't pay to join your friends at the movie, or go to the mall for a meal that is a little better than a pizza, then you are not giving them that experience. They also do not get to go home at vacation time the same way other students do. Yes, they get an education, but they also make larger sacrifices of their time and of their bodies.
The other issue is that they are 18 years old. If you take a Lebron or Melo at 18, and let them sell merchandise in their name, there has to be some sort of system that protects them from being scammed, or cheated. That could also be a concern with doing this. This may be a reason for an additional stipend instead. I don't think this stops the other problem though. The vultures will still circle and take advantage of