I would imagine if the other factions in an area don't support a particular site as being acceptable, Mab doesn't sign off on neutrality.
And by the same token, if a site is already effectively neutral grounds by general agreement, getting it Accorded is not very difficult.
In an area where no such site exists...well, it might seem nice for the ruling faction not to have any known refuge points. Until they come under attack, and find that no one is very interested in meeting with them. Not even potential 'allies'; too easy to grab hostages or take other actions (and then blame the 'enemy'). And you can't meet with the enemy, so there's no way to negotiate an end to the hostilities. (I'd bet the Accords require such meetings to be on 'accepted neutral grounds'.) So, you can't call in help, and you can't talk to the enemy. Suddenly that decision to ban neutral grounds doesn't look so reasonably.
In the story mentioned, what if the nominated party decides to reject the 'honor' of becoming Neutral Grounds? Can you force someone to be neutral under the Accords?