Why should David Cameron apologise for referring to some of the opponents of bombing Syria as "terrorist sympathisers"?

Why should David Cameron apologise for referring to some of the opponents of bombing Syria as "terrorist sympathisers"?

A couple of nights ago during the lead-up to the Syria vote, David Cameron addressed the 1922 Committee of backbench Tory M.P.'s, with the idea of quelling any dissent in his own party. During the meeting he is alleged to have made the following remark:-

“You should not be walking through the lobbies with Jeremy Corbyn and a bunch of terrorist sympathisers.”

Understandably perhaps, Cameron's remark provoked quite a strong reaction. During the debate itself, Jeremy Corbyn made fully twelve demands for an apology, in response to which Cameron stonewalled.

But should he have apologised? Well, the answer depends on how his remark is construed. Does he mean that 
(a) voting against bombing is a signifier of terrorist sympathies?
or that
(b) some of the opponents of bombing harbour terrorist sympathies?

Plainly, if he means (a), then this is quite simply wrong, and he should apologise. He should acknowledge the integrity of the many principled opponents of bombing whose opposition is untarnished by sympathy for terrorism.

But I don't think he does mean (a). A number of tories were opposed to bombing, and he certainly wasn't calling them terrorist sympathisers. He was trying to persuade them of the correctness of his point of view, and you don't generally bring this about by insulting someone.

This leaves us with (b). If we construe Cameron's remark as pointing out that some opponents of bombing harbour terrorist sympathies, what then? Should he apologise? Presumably not, if it's true.

Well it IS true.

Here's Jeremy Corbyn's own Shadow Chancellor, no less, having to apologise for having said that IRA members should be honoured:-
http://www.bbc.co.uk/news/uk-politics-34285308

Perhaps all that Cameron is guilty of is not having expressed himself sufficiently clearly. Perhaps he should have said something like
“You should not be walking through the lobbies with Jeremy Corbyn and his terrorist-sympathising Shadow Chancellor.”