As sbm12 pointed out, this provision is for FAMs - no screener, screening supervisor or anyone in that hierarchy has arrest or detention powers, nor will you ever see a screener carrying a weapon.

Hopefully once TSA is moved under DOT (a potential project of Napolitano's), the FAMs will be stripped from TSA/DHS and moved back under FAA.

Screeners only have as much power as you give them - they can search you at the checkpoint, and search you again inside the sterile area, but outside of that, you're free to tell them to go take a flying leap. I recall a few instances of screeners roaming the non-sterile area of the terminal, the public areas outside the terminal and even the parking garage and asking people to check bags, ID, etc. - if asked, just say no. They can't detain you, and by the time they scream for the police, you should be long gone.