Actually,
scs, it's quite
illegal in the U.S. -- it's a
federal crime.
18 USC § 1705 states that it is a crime to damage a mailbox or to tamper with the mail contained inside. It's federal because any mailbox is considered federal property. Other state laws may also apply; a quick Google search showed that Tennessee has a similar law, although I don't know how this would pass the double jeopardy restriction in the Constitution. Perhaps state laws were passed because the local U.S. attorneys didn't or wouldn't deal with your average suburban punk.
Source: http://www.usps.gov/postalinspectors/smashbox.htm