Rule #1 of gift giving: You give a gift with no expectation of return. (In this case the "treats"). If you can't give a gift without expecting something in return its no longer a gift, but a business transacation and you either need to not give the item or spell out all of the terms clearly.
Rule #1 of work: You judge employees on their *work* performance. Use any other personal criteria (like giving/not giving treats) and you are setting yourself and your entire company up for legal repercussions. (Setting aside of course, the morality of docking someone the raise/bonus they've earned for petty non-work issues.)