In credible dramas where this kind of thing happens, the police are willing to let the small fry go in order to catch the big fish. It's hard to see how this fits with Ronnie and Phil Mitchell though (although presumably Roxy told the police that Carl's death was self-defence so they know Ronnie isn't a murderer).
There may be more to Phil's criminal activities than previously let on to us viewers. I've always suspected so (and have mentioned this previously) because he must get his never-ending stream of cash from somewhere. We know for instance that the Arches has been involved with ringing cars - which is often done by organised criminals. However, the idea of him being being some kind of head of an organised crime ring is ludicrous...
Perhaps Phil's nemesis Marsden has convinced the organised crime squad that Phil's a much bigger deal than he really is in order to get one over on him once and for all?
I suspect they're writing the EE they want to see instead of considering their wider audience.