Although it sounded absolutely crazy that they would allow this to happen, added details reduce their culpability. On Morning Joe today, Scarborough repeated what a few of his contacts in the intelligence community told him.
The bomber was allowed into the base in his car, and got out. Three CIA agents coming to greet him noticed his hand in his pocket...they told him to remove his hand and he then blew them up. It did not happen in a room, but outdoors. And remember, this guy had given them some excellent info in the past.
It appears we are starting to hear different scenarios develop on this one. The we read about your other UK post where the Saudi Prince was blown up using similar tactics.
Probably nothing much has really changed within the Pakistani military/intel since Musharaf left.
They only cooperate to a given level when it benefits them directly in cash handouts and when going against their local Taliban and associated terrorist groups who do damage directly to Pakistan.