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They can, but the recipent will see that the package has been opened? It's pretty obvious.

Oh that's so obvious I didn't even think of that.

Depends on how badly they want to conceal the information. There's a whole discipline around opening documents covertly (flaps and seals).

There are a number of US laws that protect mail / post explicitly, and don't explicitly protect email. See this article on the matter:

http://www.slate.com/articles/technology/data_mine_1/2013/06...

One of the laws in question:

http://codes.lp.findlaw.com/uscode/18/I/83/1708

They probably don't have to for most mail. I posted this yesterday in the Jimmy Carter's mail discussion thread. tl;dr It isn't difficult (at least in principle) to read mail through the envelope. https://news.ycombinator.com/item?id=7456157