They do, and so does every restaurant everywhere. The unanimous decision by restaurants is to give you free ketchup for your food (at least, I have never seen one that charged for ketchup).
The differences:
A) many places have the big tubs of ketchup that you pump into a cup. No real incentive to steal a cup of ketchup.
B) ketchup packets are cheaper than the little sauce containers.
C) ketchup is assumed to be included with fries, burgers, etc. Other sauces are not. They're a customer specific extra (for fries/burgers/milkshakes/whatever). Dipping sauce is assumed to be included with nuggets, so you get a free one/two/eight (depends on nugget quantity).