“It seems a violation of contract for an contractee to use an outside party”
There are two sides that signed the contract. Why should one of the signers claim all right to interpret the contract?
The player’s union had the right to sue for breach of contract. The judge ruled the contact allowed for fines, but not for suspensions, and requires the NFL to comply with the contract they signed.
It’s not a violation of anything, because in the end that’s a large part of what civil courts are for. A significant percentage of the cases that land in civil court involve 2 sides disagreeing on the meaning and application of a contract. And notice that the ruling was based on both previous legal cases involving the league AND results of their own arbitration and appeals process (Tagliabue’s throwing out of the suspension of players for not complying with the Bounty Gate suspension was specifically sited). He strongly encouraged them to find an agreement multiple times, they failed, so one way or the other he had to rule. The only question at that point is how would he rule.