from a legal standpoint this isn't a violation of search and seizure.
Once you step on board an airplane (or into an airport) you sign away alot of rights including speech. For example you can't say 'hi' to a friend named Jack.
I think the constitutional reasoning behind this is the "congressional power to regulate interstate commerce" clause.
Also I think it is generally viewed that being aboard an airplane you are completely puting your safety in the federal government's hands and as a result they are completely in charge. .