They are the only ones who swear to tell the truth, hence you can't ask questions of Congress while testifying.
Howard Hughes stood up, told them he'd had enough of the circus and walked out. They can compel you to appear but they can't make you stay. Kind of a strange arrangement.
Didn't Cock Cheney say when he was asked to appear before Congress and tell the truth: "Fuck off, I'm the Vice President."