The Patriots owner accepted the penalties, but he could have done it differently.
New England Patriots owner Robert Kraft accepted the DeflateGate penalties because he had a near-zero chance of winning an appeal. There were three courses he could have taken, and redditor trout45 did a good job explaining the outcomes (I've edited for formatting and, ahem, language):
1) A federal arbitration claim: Kraft would have to prove the discipline was excessive and arbitrary and that the NFL didn't correctly apply it's own rules. This would be an incredibly difficult standard for Kraft to show and would likely fall flat.