Federal law/recent SCOTUS decision

I have a simple question about a recent SCOTUS decision.
Today that SCOTUS upheld a new federal law that makes it a crime to
send messages over the computer that offer or seek child pornography
even if no such pornography exists (link 1 at bottom). It has been
known for awhile now that the FBI has been posting hyperlinks that
purport to be child pornography and prosecuting those that click them
(link 2 at bottom). TITLE 18 § 2252A(a)(3)(B) (link 3 at bottom) from
what I've read has no exemption for law enforcement officers. So the
obvious question is isn't the FBI agents participating in these
'stings' in violation of US law and subject to no less than 5 years in
prison?

link 1:
http://www.latimes.com/news/nationworld/washingtondc/la-na-scotus20-2008may20,0,6024926.story
link 2: http://www.news.com/8301-13578_3-9899151-38.html
link 3:
http://www.law.cornell.edu/uscode/html/uscode18/usc_sec_18_00002252---A000-.html