The National Football League has made it clear over the years that it is a business. The problem is that the NFL is a messed-up business, especially when compared to other sports leagues.
Football players are some of the toughest, most physical athletes in the world. They're subjected to more physical abuse than athletes in any other sport. Yet football players tend to have less financial security than top-notch athletes in other sports. NFL contracts aren't guaranteed like they are in the NBA or MLB.
If an NFL player underperforms, he's cut or is forced to renegotiate his contract.