It gets pretty hard to know what TSA's mission is these days. Seriously.

They patrol parking lots. They do VIPR operations and stop vehicle traffic for inspections. They direct exiting train pax away from a normal exit and through a 'sterile' area to force screening on them. They show up at train stations, sporting events, bus stations. They inspect rental cars being returned.

(That's aside from the ones who try to extend TSA's mission during their off-hours - you know, to 7-11's, or to hurry traffic along).

If TSA can stop me in the airport parking lot or the landside area of the airport and question me, how do I know where their responsibility starts and stops? If I'm getting grilled on the landside area (ironically, on a day when I'm not even flying) and BDOs routinely roam the landside area, then how am I supposed to know that TSA is accepting responsibility for security in the non-sterile area by questioning me but TSA is not accepting responsibility for abandoned bags or suspicious items found lying around.