At my airport, the TSA screeners are required to check the boarding pass (since the ID checkers don't check for SSSS). We also have to initial it, so they can track us down if we miss a SSSS. For me, personally, it is easier to initial it while supporting it with my palm. If a passenger had some objection to this, and wanted to hold onto it him/herself, I would have no problem initialling it in his/her hands as long as I can positively determine that s/he is not a SSSS.