I always wonder, too, if all of those people think to leave the packing slips out.
I think the idea is that US Customs (or gov't internet snoopers and Postmasters) probably don't invest a lot of energy into this sort of thing because the package is leaving the US. For packages coming in, I'm sure it's another story. That's just a guess.
There's a thread on this very subject going on at the eBay boards right now, actually. I can't remember which board -- maybe the packaging/shipping one.