One of the most surprising events of a pretty eventful offseason in the NFL was when the news broke just days before the start of free agency that the NFL was stripping huge amounts of cap room from the Redskins and Cowboys for violating largely unwritten rules about frontloading contracts in an uncapped year. The Redskins lost $36 million in space over two seasons, while the Cowboys lost $10 million.
The two teams filed grievances against the league (of which each are 1/32nd owners of) to appeal the decision. Today, an arbitrator ruled against them, dismissing their claims. Afterward, spokesman for each team said they accepted to ruling and would abide by the arbitrator's decision to dismiss.
During the uncapped year, the owners more or less agreed to not "frontload" contracts and to treat the year like they would any other. The idea was that teams wouldn't put a ton of money in a contract up front in the uncapped year, so they could then save space later in the deal when presumably there would be a salary cap. It also protected the less cash rich teams from being outbid for free agents with huge amounts of upfront money.
The Redskins and Cowboys ignored the warning/agreement/whatever you want to call it and this season the other 30 owners made them pay. They voted unanimously to strip the two teams of the cap space they would have used up with the large, front loaded contracts in the uncapped year.
While it would seem that the Redskins and Cowboys really didn't do anything legally wrong, they also really didn't have much of a legal standing to challenge the actions of the other owners. The union certainly would have seen it as collusion, but as part of their settlement with the NFL, they agreed to give up any right to sue after the fact. Also, the cap space taken from Dallas and Washington was distributed to the other 30 teams, so the players didn't lose any money in the deal. Also, since the NFL is co-owned by the owners of each, it really has broad power to do what it likes as far as dealings between them.