Possibly because he is not "Leader of the Opposition". When Corbyn took the job - voluntarily - did he really expect not to have his background mentioned? If complete fabrications like the pig story can be used in one direction, rather more relevant indiscretions can be used in another.
That's not a reasonable comparison because the pig story has never - to my knowledge - been used by the Labour leadership or the Labour Party as a means to attack Cameron. I seem to recall at the time that it was reported that Corbyn deliberately avoided any mention of the story because he didn't believe that kind of personal attack should form any part of politics (though that's a vague recollection, I don't have a link to confirm it).
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May I be allowed to pass comment on those statements that talk in terms of a negotiated peace process in the Middle East conflict areas by asking if those people supportive of such aspirations can lay their hand on their heart and say that they truly believe that ISIL would espouse such a position.
I don't get the impression that most of those who talk about a negotiated peace process expect to include ISIL in that process. Certainly, at Labour meetings I've been to where this has been discussed, those advocating such a position seem on the whole to be thinking more along the lines of a negotiated peace amongst the other groups in Syria (Assad, the pro-democracy rebels, etc.) which would then allow a more stable Syrian Government to defeat ISIL.
It is notable that, throughout his career, Corbyn has been keen to "talk" to opponents of established states, but not to embrace the establishment. To do so once may be forgiven, to do so on so many occasions suggests a certain sympathy with that kind of group.
I think your observation is correct. However, a common characteristic of many of the groups Corbyn has talked to is that they often represented (or claimed to represent) groups who were arguably marginalized or discriminated against. So, without knowing what's going on inside Corbyn's mind, it's perfectly plausible that any sympathy is for the aim to assist those marginalized people, not with the terrorist methods adopted. Indeed, there is a very strong tradition within the Labour Party of wishing to support groups perceived as marginalized or subject to discrimination, which would suggest that interpretation is very plausible.
So it's not really logically possible to deduce from the fact that Corbyn has tended to talk to such groups that he today supports terrorism. Indeed, the fact that Corbyn seems to very strongly err towards always seeking peaceful resolution to any conflict, would seem to be completely incompatible with such a claim. And remember, that Cameron's description of Corbyn (and unidentified other people in Labour) as a
terrorist sympathizers carries strong connotations of current (not historical) direct support for terrorism - a dreadful claim for which there is no significant evidence (at best, all you have is a vague association based on having met some groups in the past). That certainly doesn't seem to me like the sort of claim a (supposedly) responsible Prime Minister should be making.