Maybe they are using the TSA to bypass search restrictions within the US? I know customs can open packages without a warrant issued by a judge but once it is inside the boundaries of the US it is officially US mail and there has to be a warrant issued. That is, so long as it was mailed using the US Postal Service. UPS, FedEx, DHL aren't obligated to comply because it is legally considered their package until it is released to you so they can open or authorize it to be opened by LEO.
That is interesting, the more I think about it. I am going to do some research.