You think that if a player is running and in full possession of the ball and a defender puts him arm across him and initiates contact without being anywhere near the ball, that isn't a foul?
Blimey.
I have no sympathy for us or Kos. The ball that was played into Lewandowski was a simple little through ball that gets cut out 99 times out of 100. It took three Arsenal players out of the play including Kos, who had stupidly stepped up. Any defender who puts his arm across a player on the ball, in the box, has no complaint if the ref gives a pen.