Forget about Costco, if some people here are so convinced this behavior is illegal they should be going after every fast food company that offers anything like "get a free/cheap xyz with any drink purchase!" Where the subsidy is obvious.

Costco gets to sidestep a lot of regulations because they technically are a private club with paid membership. The US anti-monopoly laws are also unusually weak.

In other countries, selling a $7 chicken if it's subsidized by the sale of other goods can indeed be illegal.