A quick google search for "USPS stealing mail" reveals plenty of results, including news stories about USPS employees stealing mail.
Of course it happens, it happens in all postal systems. However, the difference is that USPS do something about it. From a news article about a USPS employee stealing mail:
The U.S. Postal Service received complaints from residents who suspected Dillard had taken their mail, according to court documents. USPS investigators planted mail that should have been delivered on his route.
On Oct. 8, 2010, they stopped Dillard after he completed his shift and found an opened birthday card that had contained a $20 bill that had been placed by the investigators.
ongo.com/v/2513130/-1/56457B72CB243878/ex-postal-worker-who-buried-mail-in-his-backyard-sentenced
Royal Mail do the same thing but they even use agent provocateurs, as a friend of mine nearly found out to his cost.
Go to Poczta Polska and complain about your post being stolen and see what they will do about it. Not so long ago there was a TV program which showed a video taken from inside a PP truck showing workers sorting through parcels to see what to steal: none of those workers were even officially reprimanded by PP.
