Needless to say, this is a gross abuse of power; the AP couldn't even challenge the seizure in court because they executed the seizure secretly. It's particularly frightening because it dissuades anyone, particularly government officials potentially privy to corrupt programs & practices, from anonymously talking to journalists.
http://www.nytimes.com/2013/05/20/business/media/head-of-the...
EDIT: here's a Wikipedia article on the topic: http://en.wikipedia.org/wiki/2013_Department_of_Justice_inve...
The US does not systematically persecute investigative journalists, at least not yet. But the sentiment is there, and there are cases that have crossed the line.
http://en.wikipedia.org/wiki/Barrett_Brown
http://falkvinge.net/2013/06/30/with-journalism-persecuted-t...