Please note that you're probably referring to the boarding pass check. Identity documents are only checked when passengers choose to present them. Best we can tell, this is optional. It seems that most people choose to do so, partly because TSA staff frequently misrepresent this option as a requirement, and partially because doing so almost guarantees that one will be searched less thoroughly and will be required to answer fewer questions from TSA.

See also: Flyertalk: TSA's airline passenger identification policies