Instead of having customers donate a dollar and giving them a coupon book "worth" $30, why doesn't KFC just donate those $30 themselves (or even half)?
the others have described things well so read them first!
the irs looks at large charitable donations with suspicion because it is easy for a charity to pretend to do good but really just things the donars would do anyway but now they can deduct it. Thus large donars often have to do a match of smalaer donars as the large donation with no small donars suggests only the rich care and so maybe it isn't really charitable.
remember the above is a factor in some cases but shoud be considered only after you exhaust the other reasons to do this and find they are not enough.