They haven't since there is no proof.
The train of thought is not really difficult:
a) Xe is a militant outfit known to commit atrocities
b) Suggestion is that they are taking part of counter-insugancy operations
c) Such operations are not really done out in the open
Absence of proof is not proof of absence.
Now, be a dear and tell me how my original claim, (that this is "plausible", given the history of Xe), is somehow absurd compared to your flippant "can't happen" stance.
You're point regarding applying this illogic to Muslims is moot, not to mention malformed; you're the one exercising this reasonin, not myself. To put it in simpler terms, are you prepared to extend this weak reasoning to terrorist activites with plausible denial?
To spell it out even more clearly for those who have issues with comprehension, if there is denial of involvement in particular attacks blamed on terrorists, will one demand proof, or rely on past history of terrorists?