To be fair, the TSA website does say that "If your identity cannot be verified, you will not be allowed to enter the screening checkpoint."
https://www.tsa.gov/travel/security-screening/identification
https://www.tsa.gov/travel/security-screening/identification
"In the event you arrive at the airport without proper ID, because it is lost or at home, you may still be allowed to fly."
Every time I've done it, they ask me a few questions and I get a pat-down which is no big deal anyway since I never go through their scanner anyway.