TSA has a statutory mandate which is pretty much WEI only. Some screening of people, too. Adding Marijuana to the list of prohibited items no more expands their statutory mandate beyond WEI than adding pink striped socks to the list of prohibited items. But an agency exceeding its statutory mandate is not necessarily a constitutional violation, and that makes it all the more difficult to detur agencies from exceeding their statutory mandates.

United States v McCarty gave TSA a pretty good roadmap of how to conduct themselves so as to make any potential evidence 'discovered' during screening admissible. But if TSA had to defend putting Marijuana on the prohibited item list in proceedings under the APA, it wouldn't be an open and shut case.