The TSA may have a duty to protect the traveling public against threats to transportation, but that does not include a duty to protect an individual from an illegal act committed by another individual.
Ahh, but numerous TSOs have told us that if they have evidence of a crime right in front of them (drugs, child porn, etc.), they are duty-bound to act. Since they inspected the imposter's identification, they should've discovered the crime in progress and acted.
If that's the standard that TSA holds for itself, then they've got to live with it when they fall short of that standard.
[Obligatory aside: this is not an endorsement of TSA policy in such matters. But that's another thread.]