No where is there anything that say that is where the money goes. We all hope it goes back into development but that is not always the case. Sometimes it just goes into more cash shop junk development. Look at CO, they have had a cash shop all along and haven't seen any new content in over a year.
Also, just because something is "not required to play" does not mean it isn't important to peoples enjoyment of the game. If we stripped away everything that wasn't needed to play the game it would be a pretty dull and lifeless product not far from a game of Pong.
And if we stripped away all revenue then we wouldn't have a game at all.