The lowly TSA hourly employee was probably shaking at the thought of having to do that to a US Senator and possible consequences but he had to do it.
Powerful people have the option to work their connections to end the TSA, but that is a lot more effort than paying the $100, giving up their fingerprints and accepting the mostly hidden indignity of a background check (which, for $100 can't be more than a credit check, real background checks cost tens of thousands of dollars). So the TSA has basically taken the wind out of the sails of anyone who could hurt them but is not ideologically opposed to what they do, and frankly, the vast majority of Americans, rich and poor, haven't thought about it enough to have an idealogical position and will be happy to take the path of least of resistance.
As for all the "regular" people who sign up for pre-check, they don't in anyway negate pre-check's ability to co-opt the ire of powerful people. If some percentage of the plebes sign up, that doesn't make things any more annoying for the powerful. Most regular people won't be signing up, if you don't fly more than a couple of times a year, it usually isn't worth the effort - you get past the checkpoint and just try to forget about it while you go on with the rest of your trip.
They might if they had to put up with the same crap the plebes do. But the TSA has figured out how to keep all the powerful people from having to undergo the same indignities that normal people do - they let them opt out of almost all of it for a background check and about $100.
http://www.tsa.gov/tsa-precheck
Which, I'd like to point out is ridiculous if you believe that the TSA is about actual security. They've just created a whole class of passenger that get to bypass almost all of the 'security.' Background checks don't prevent people from being conned into carrying explosives in their baggage.