The entire point of much of the legislation requiring monitoring of the communications of bank officials is to combat insider threats (though not to security in the usual sense). They're required to monitor all communication (i.e. not just official e-mail that can be easily monitored another way) so there is documentation if any of their employees does something funky (fraud, etc.).

In this case, the endpoint by necessity is physically accessible to the possible adversary, which means they have a whole host of methods for disabling monitoring software. It's much harder to interfere with a box which you don't have physical access to that is listening in on your communication, and which simply drops any data it cannot intercept.