Is not that simple mainly because both sides are using politics to disrupt the cricket.
PCB are threatening a pull out because BCCI are not touring or playing Pakistan in December and are basically using the excuse of them not allowed in Mumbai to cancel it.
And in india the main government don't want India to play pakistan in the foreseeable future.
Apart from Mumbai and Shiv Sena strongholds Pakistan could travel to india safety and Aleem Dar, Akram, Akhtar and many more have been in India so many times in recent times with no security issues, they even had no issues in Mumbai.
This was a publicity stunt by Shiv Sena to cancel the series and PCB are mostly pissed because it was planned and Staged affair with Camera present and BCCI basically letting it happen and wasting PCB time.
ICC got a very hard job on it hands considering both parties are playing games and will give a bad name to the sport.
If they cancel or move the tournament ICC will lose big bucks and would possible cause issue considering india is scheduled to host 2021 and 2023 ICC events and will give PCB more opportunity in the future to cause mayhem if they don't get their way.
They play the tournament without pakistan then the entire tournament will be under the shadow of no pakistan.
Essentially this article explains why pakistan are doing this.
http://www.telegraph.co.uk/sport/cricke ... llion.htmlPCB feel it was conned in signing the big 3 by BCCI/ECB/CA and essentially the only actual board that has the power to rebel against it.