I suspect it is because the TSA is not only adding much (if anything) in the way of adding incremental security protection, they are doing harm in two ways:

1) diverting limited federal resources that could be more effectively allocated towards targeting potential terrorism threats; and

2) generating huge "frictional" costs on the U.S. economy via increased "all-in" travel times and costs.

A tertiary issue is what appears to be a significant squandering of funds that are allocated towards their mission, whereas a final criticism appears to be their diversion of resources to missions which are outside their defined scope in an apparent attempt to create more momentum for additional federal funding.