Best question I've read in a long time. Please take a seat at the front of the class. ;^)
'Cause the Constitution protects their sorry selves?
Article I, Section. 6. The Senators and Representatives shall receive a Compensation for their Services, to be ascertained by Law, and paid out of the Treasury of the United States. They shall in all Cases, except Treason, Felony and Breach of the Peace, be privileged from Arrest during their Attendance at the Session of their respective Houses, and in going to and returning from the same; and for any Speech or Debate in either House, they shall not be questioned in any other Place.
Support and Defend applies to all of it.