They feel the NFL is rigged because: 1. refs ALWAYS get blamed by a large portion of fans for every outcome, 2. conspiratorial stuff is an easy, mindless way to approach outcomes, 3. TV, and TV replay give fans a better view of everything than officials often have, and 4. There are so many questionable actions by players on just about every play that it's easy to pick out instances where you "got robbed" because refs have to make judgement calls all the time about what actions are REALLY penalties. If they were able to see and willing to call, all the actual penalties that happened on a play, you'd never complete a down.
Not much different than hoops, where they allowed carries, travels and physical contact to the extent where, now, to call something, 20%+ of fans are going to think each call is wrong (either because they think it's not a foul, or because they think some previous action should have also been a foul if that one was).
But no one would like the product if they called all these fouls/penalties ... so we have this endless spiral of "we was robbed!"
Again, the Sanders catch & fumble/incompletion was a more significant play with at least as much murkiness as any other particular play that happened, and that went in Philly's favor (after review, of course).