It has nothing to do with the carrier, it has to do with Customs.
UPS, FED-EX, DHL, Airbourne, USPS - all have to cow down to U.S. Customs. DHL, UPS, and Fed-Ex actually have Customs/DEA guys walking through their receving centers with their sniffing dogs. My Fed-Ex deliverer works for me on his day off and tells me the stories.
So, the moral of this story is that Customs is really the bad guy when it comes to seizing. The good thing about Fed-Ex is that they are on such a tight time schedule that Customs doesn't have much time to decide what to snoop through. A lot of stuff gets through just because of the how fast they move packages.
When my friend asked one of the DEA guys what they look for in the packages, he was told that they look for packages with a certain weight coming from a certain area. Also, suspicious smells were dead giveaways.