I'd allow the sponsorship, because in many cases it's a large percentage of a fighter's income. The more comfortably the fighters are living, the less likely it is that they'll come together and start demanding more pay.
Aside from that, sponsors on shorts/shirts/hats are no threat to commercials. I don't see 5 minutes x 3 (or x 5) exposure of a logo that's on the side of a guy's shorts being any real competition to a well-designed 30 second commercial. The logo is constantly moving, being covered, twisted, etc. To say nothing of the fact that there's a good chance the guy wearing your logo will lose the fight. There's an aspect of uncertainty that you don't have with a commercial, which has a predetermined outcome, whose execution was meticulously planned by professionals in a boardroom. There's still way more incentive for a business to do a commercial over a sponsorship (although both are obviously better than just one or the other). There are decades of research about the effectiveness of television commercials; there isn't such research on MMA fighter sponsorship.