Correct me if I'm wrong, but I think the issue is not so much what he said (which in any case makes him a tw@t) but rather the fact that he's one of the thin blue line that's supposed to be protecting us from the bad guys and that he - more than anyone - should be aware that (a) it's not funny and (b) an offence under the Aviation and Maritime Security Act.
I note too that GMP - who are not infrequently a dirty dark grey shade when it comes to matters of honesty and probity - are saying that the officer concerned will not be prosecuted. Why not? I agree that it's a relatively minor thing but in this case it's the principle.
Send him down!