For me, it's just adding an unnecessary company in the middle of a transaction.

When I buy something, I accept that there's somebody selling something, and frequently a credit card company facilitating that transaction. Why do I need Facebook to know what I'm buying when purchasing things has already been made easy-enough for most cases? With Facebook inserting themselves into the checkout process, I'd be giving them something of value without really getting anything for it - so why would I do it?