I don't know what other countries are on that list, but I think places like Nigeria should be at the top of the list. It's unfortunate for a lot of honest people all over the world that others in their country aren't so honest, and their governments don't do a whole lot to crack down on the handful of people who are problems.
I don't know what the specific reason is that Paypal is denied to the citizens of Pakistan, but it's not as simple as Paypal is at fault here. Pakistan fully has the power and authority to "fix" the reason for Paypal's decision. If Pakistan chooses not to, or is unable to, then, well, blame your elected officials.