Or, if you prefer, Who will guard the guards themselves?
I believe that this is an illegal and unwarranted intrusion under the Constitution. Others have addressed that, here is my alternate path to ending nude scans.
Corruption exists at all types and levels of Federal Law Enforcement. I think we all know this without really understanding the depth and breadth of the problem.
In 2004, the US employed something around 105,000 Federal Law Enforcement Officers who are exempt from screening when boarding commercial aircraft.
In 2005, in ONLY California and Texas there were SEVENTY-FIVE cases of corruption in border and immigration alone. 
These exempt FED LEOs are not all FBI or ATF or Air Marshals, they come from such groups as the National Marine Fisheries Service, the Department of Education, the Department of Commerce, and even the Office of Surface Mining Reclamation and Enforcement.
Granted that the vast majority of these men and women are honest, honorable, law abiding and well-meaning people, "vast majority' is not 100%. Hundreds (Thousands? All?) of these Federal LEOs are susceptible to corruption, bribery, coercion, blackmail, identity theft or simple insanity, and yet each and every one strolls up the exit, flashes a badge, and boards an aircraft with whatever they choose to carry onboard.
Consider this scenario, and decide for yourself whether it is likely or only possible.
Agent Broke (not a real name) thinks that US Immigration Policy is unfair to Latinos. Agent Broke takes money to ignore certain document irregularities in an attempt to personally correct that policy. The people who pay Agent Broke, tell him that they need a package delivered to a man waiting for flight 773 inside the San Diego Airport terminal, and they give Broke a small package, a plane ticket for a different flight and $10,000 in cash.
Agent Broke is stuck. He has been working with these people for a while and coming forward with this will expose his actions and cost him his career, retirement and freedom.
Maybe the package is drugs, maybe the package is money or fake ID, maybe the package is C4 or a bio weapon. The point is that the package is completely unchecked and we will never know it exists.
There is no logical or necessary reason to exempt these FED LEOs , or anyone else. If it is proper and necessary that they be armed during flight, fine. They can be nude scanned, and present the documentation allowing them to be armed; BUT WE have the right to KNOW that they are only carrying weapons and articles that they are legally entitled to carry.
The same logic applies to any exemption to the rigorous, and unreasonable, screening. Anyone who is exempt, even if they do not fly, can pass items to persons who do.