Rumors we hear in the industry are that the oval track cartel gives rebates to sponsors through the back door such that in some of its divisions a sponsor gets almost a free ride. While this rumor may have no merit, if true it accomplishes two things – 1) It makes it look like the cars are sponsored and the series is successful to lure in other sponsors, 2) It keeps potential sponsors away from competing series making it hard for them to land a paying sponsor when the competition is giving it away virtually free. Because the oval track cartel has huge amounts of money flowing in from other sources, the rebates are nothing more than a tax write-off until such time as the competition starves and dies off.
Just another conspiracy theory, though one that may have some merit. For example, how else do you justify all those sponsored cars in the Grand-Am Series with near-zero attendance and TV ratings? The economics just don't work unless the sponsorship is dirt cheap.