I think because they believe there are good FBI agents who do good work enforcing federal laws. The Sean hannity view that most FBI agents are good. That's probably what most people believe.

I think most of the laws are unconstitutional and illegal and therefore the people enforcing illegal or tyrannical laws are not good either. But the average FBI agent probably believes in the great good they perceive themselves as doing, the feeling of pride in being public servant or believing in ones oath to uphold laws without using discretion that the politicians and judges are corrupt and the laws are corrupt therefore enforcing corruption is corrupt. Much about subjective perception