In no other country can a person work 16 days a year for 3 and a half hours each of those days and get paid 100 million dollars.
And yes, they are ALLOWED to play in the NFL. Many people bust their ass much more than any NFL player to get hired on at a great company for a great job, and it's still at-will employment. The company can fire them at any time for any reason or no reason at all. Same with the NFL. They do have contracts, but there are many reasons they can be let go. They are voluntarily accepted to play in the NFL. They have no RIGHT to be in it. So yes, it is at-will employment.
Just ask Michael Vick.
If you think playing in the NFL is just 3.5 hrs on 16 days, you know absolutely nothing about what a professional athlete did to get there and does to stay there.
And how many jobs out there virtually guarantee you will be injured? If employees were injured on the job at the rate NFL players are, OSHA would have shut it down years ago.
And as for what it takes to get hired by the NFL, there are probably 10 years of busting your ass and training virtually nonstop before you even get an interview. Yes, the NFL can get rid of them just like any employer. But the value of the athlete is different than an employee at a normal job. The normal job employee produces the product. In the NFL, the players ARE the product. And they are paid accordingly.