According to NFL spokesman Brian McCarthy, "every dollar of income that is earned in the National Football League -- from game tickets, television rights fees, jersey sales and national sponsorships -- is subject to tax. None of this income is shielded in a tax-exempt entity. Instead, the NFL's 32 clubs pay tax on all of these revenues."

But that's not 100% true, Coburn said.

"The individual owners and teams pay taxes. We're not going after them, but what they do is they can put all this confluence of money into the league office and do this as a nonprofit, which means they're not paying taxes like every other business that would be in a trade business like they are," Coburn said.