That's because one of those things is a return and the other is a trade in. Would you prefer that Gamestop didn't let you return used games in full?
Edit: That reminds me, way back when I worked for Gamestop I was always amused by the customers who would try to return a game with the excuse "But I didn't like it". In those cases I would remind them that we were not a rental service and if they would like to "try before they buy" than they should go to Blockbuster first.