Because there is no obvious target to retaliate against, and because there are not enough police to police every opening in an open society, either the Muslim world begins to really restrain, inhibit and denounce its own extremists - if it turns out that they are behind the London bombings - or the West is going to do it for them. And the West will do it in a rough, crude way - by simply shutting them out, denying them visas and making every Muslim in its midst guilty until proven innocent.
And because I think that would be a disaster, it is essential that the Muslim world wake up to the fact that it has a jihadist death cult in its midst. If it does not fight that death cult, that cancer, within its own body politic, it is going to infect Muslim-Western relations everywhere. Only the Muslim world can root out that death cult. It takes a village.