@reasoning logic,
The U.S. Congress is not subject to the civil service, equal opportunity or other labor laws it has passed. Same goes for other laws. Until very recently the Congress operated an archaic coal-fired steam generating plant in Southeast Washington, not far from the Capitol, to provide (originally) heating steam and electrical power to that building and the original office buildings for the legislators. No attempt was ever made to have it comply with EPA regulations, and the EPA never dared attempting to enforce its policies on the body that created it. (My company provides engineering services to the Architect of the Capitol who ran the plant). Congressmen are even immune from arrest for nearly any crime while Congress is in session.
In short the legislature is an independent branch of government. It is subject only to its own rulles and the Constitution, particularly on matters relating to its legislative activities.
There are ample legal precedents for all this both in Federal and state law. The preliminary ruling of the Wisconsin court overturning the legislature's action violated all of them.
That, in short, was also the substance of the Wisconsin Supreme Court Ruling on the matter. You can easily find and read it.