Because the McCanns have obviously not involved the police in the manner that Lee Rigby's parents obviously did.
I'll ask this again as it's relevant I think - the Daily Mail has proclaimed that Spivey is a troll - what do McCann "sceptics" think about this, and about him and his activities?
From what I can gather, the general idea is that free speech rules in the name of "democracy". If you were to claim that someone is a paedophile, when actually your literacy skills didn't extend to realising that that a paediatrician is actually a children's doctor... then that's still free speech. If some nutjobs decide to take vigilante action and firebomb and kill the doctor's family, then the fault is on those who assumed that the spoutings of the "truthseeker researcher" were accurate.
Numerous murders have been started by such people who appeal to the outraged but barely literate willing to take "action" in the name of "truth" and "justice".
We're supposed to be in an enlightened age... but sometimes it seems as if we're back to the middle ages.