You seem to be under the impression that the word "screening" means TSA can do whatever it wants. When in fact, if you click on the link under the word "Screening" in the own link you posted, there is a definition provided.
> (4) Screening defined .— In this subsection the term “screening” means a physical examination or non-intrusive methods of assessing whether cargo poses a threat to transportation security.
> (4) Screening defined .— In this subsection the term “screening” means a physical examination or non-intrusive methods of assessing whether cargo poses a threat to transportation security.
You should consider reading what you posted.