On paper they are told to do a mission and then they are undermined every step of the way, even by their own leadership. They have the lowest morale of any workforce within the government, and for good reason.
Not to mention that CBP has two virtually exclusive missions. We have to let the good guys in as quickly as possible, while keeping out every single one of the bad guys.
Throw in the fact that the good guys sometimes look like bad guys, and that the bad guys are trying to look like good guys, and you can see how difficult the CBP mission is.