I'm sure Dale Johnson will go over the incident in his weekly var/decisions thread on twitter but from what I understand you only have to wait for the whistle if the ref has told you to and this generally only happens when he's measuring out a wall or booking somebody. That said, it was slightly surprising that he allowed the free-kick to be taken quickly as he didn't appear to be looking when it was taken but that's Jon Moss for you.
I know a lot of people have or will compare it to the Brighton disallowed goal the other week but as above, because a wall was being lined up that's why Brighton had to wait for the whistle. And it was because the ref noticed (after blowing his whistle the 1st time) that the Brighton keeper was still lining his wall up, that he blew again to stop the game.