It's very simple really - he's a UK citizen and as a UK citizen he has a right to return to the UK, a right to claim benefits where applicable and a right to free speech including criticising Britain.
I gather that the intell services consider him a low threat a bit of a Walter Mitty character. They will have approved his access back into the country without detention because they want him to be here - presumably it's cheaper to pay him £52 a week to stay here and be watched than spend thousands sending someone to Pakistan to watch him. . .
so how does a known terror suspect gain immigrant status in the UK with this kind of a past history? Also no one has criticized his rights and liberties just that you allow someone with this past history into the country
Keep your freinds close and your enemies closer.