I don't have a problem with commercial sponsors. Our school hands out the Barnes & Nobles summer reading program forms at the end of the school year. My dd reads a bunch of books and is rewarded with a free book. The kids are rewarded for doing something we want them to do and Barnes & Nobles gets the parents through the door, publicity, and whatever else they get out of it. It's a win, win, win situation.
But I completely disagree with using junk food as a reward. As a society we're overweight, under-exercised and have unhealthy attitudes towards food. I think the program could have picked a much better sponsor. I like OHGrandmas suggestion of a book.
