So why allow them back?

The Syrian civil war is none of our business despite the attempta by governments in the West to try and get us involved. There is simply no desire amongst the UK people to fight for the barbarians of either side, civilian casualties or not. One of the problems we do have is that islamoloons from the UK are going over there to fight, which if they are killed is a good thing, the bad thing though is that they get even more radicalised and then want to come back...
Telegraph.

The crisis in Syria has emerged as the biggest threat to Britain’s security, The Telegraph can disclose.
The threat to the UK from returning fighters from the Syrian civil war is now the same as that from al-Qaeda terrorists in the borderlands of Afghanistan and Pakistan.
The increased risk will refocus attention on the decision by David Cameron - backed by MPs in the House of Commons - not to intervene as the Syrian conflict worsened last August.
For the past two years, British jihadists have been able to gain access bomb and weapons training as well as further radicalisation.
There are fears that British men who have been radicalised there are also being encouraged to return to the UK to carry out attacks here rather than staying to fight.

You'll note the slight attempt to rewrite history there by claiming Cameron backed by his MP's not to intervene when it was actually a commons revolt against Cameron which forced him not to intervene as he was all gung ho about joining in.
But doesn't it strike you as odd that the government simply doesn't revoke the passports of these people? After all if they love Syria enough to fight for the place then it strikes me that they can bloody well stay there, it's not like they will be model citizens when they return. No doubt they'd scream 'human rights' if we blocked them, but frankly as non-citizens we don't have to care, let them be someone else's problem, we really do not want a spate of Lee Rigby style barbarism hitting the UK.
The civil war in Syria is none of our concern, the idiots who go from here to fight are, doing so ought to mean a revocation of their passports and no return allowed, save perhaps those who go to give medical aid. Bit those who go to fight? Keep them out, we don't need them or their radicalised religious views.