Legally, the issue is that NFL teams are individual businesses but they function as a cartel under the umbrella of the NFL when it comes to TV deals, merchandising licenses, etc.
This would never be permitted in any other industry. If all the residential contractors in your area got together and established fixed rates for various home repairs, or all the auto mechanics colluded to fix rates for auto repairs, they’d be sued out of existence by the U.S. government.
Although it’s not really as grey as you’d think. This was actually all settled by the Supreme Court in ‘61 (maybe ‘60) when somebody sued the NFL over selling TV contracts in a block. The Court it made perfectly good sense for sports leagues to be able to do that, yes they’re kind of a monopoly in that there’s only 1 of each league, but in the long run it’s better for the teams and fans for league wide contracts to exist. Then of course Congress chimed in with the Sports Broadcasting Act. But from everything I can tell that’s really just Congress pretending they have power.
And of course one can argue if they’re REALLY a monopoly. Especially these days with other pro football leagues and the college football moving further and further away from the veneer of amateurism (and this is similarly true in all the other sports). When you get right down to it the only the NFL is a monopoly in is being the NFL. And everybody is a monopoly in that.
Funny you mention auto repair. Reminds me of the infamous Ford timing belt wrench. There was a time there in the 80s and 90s where Ford had stuck the most important bolt for changing a timing belt in a real PITA place and you pretty much could only get to it if you used a wrench they sold (at serious $$$) specifically designed (and patented) to get to that bolt. So you could only that very important piece of maintenance done at Ford “approved” shops. Of course this turned into one of the many reasons to not buy Ford since their timing belts were more expensive to replace than anybody else.
In the long run this suit is probably going nowhere. Between the SBA and the old Court ruling the NFL, and the other leagues, is allowed to do this. And if they really have to the “what exactly are we a monopoly in?” defense is there for them, if the timing is right they can turn on a UFL game as part of their defense.