Being the most influential of all the terrorists/paramilitaries in Ireland they do set the pace. If they say it's over it is.

I imagine the thinking would be along the lines that, were the IRA to cease to be a paramilitary force, it would cease to have that kind of influence; and an alternative republican paramilitary force would move into place, especially if there were any sectarian violence, perhaps associated with loyalist paramilitaries. That's what happened with the Official IRA all those years ago.

I'd also think it highly likely that the IRA will be coming down on any unauthorised bombers, most especially any from within its own ranks, and probably quite effectively. "Quite effectively" doesn't mean 100% effective.

And it's probably more complicated than that, with bluffs and counter-bluffs and dirty tricks and all.