The law in this country must apply to all its citizens, not just one group or another. For reasons we can all guess at the Police and CPS repeatedly shy away from prosecuting Muslim extremists who carry placards calling for the death of all jews or even worse slogans.
The latest example is Anjem Choudary who said anyone insulting Islam would be "subject to capital punishment".
I accept there is a legitimate debate to be had about where the boundaries of free speech should be drawn, but with the law as it stands such utterances are clear breaches of it.
Sooner or later a Christian is going to stand outside a Mosque carrying a similar banner. I think we can all guess what the reaction will be of both local muslims and the Police.