“But the NFL is officially a “not for profit” enterprise.”
The NFL itself. Not the teams. “Not for profit” does not mean it is a charity, but rather that it nets zero. Anything “left” is not kept, but passed on to the teams where it is taxed...so there is nothing to tax except money passing through, which would be unreasonable.
They get the lions share of their money by sales and not by donations. They negotiate deals and contracts for money, they sell merchandise and/or license merchandise for sale, they research means of greater revenue, and they take a portion of the proceeds for their personal and organizational funding.
Just because they spend all their profit doesn’t make them a non-profit.