I'm sure it has happened in the U.S. that a prosecutor has given disinformation to the press, but "normal"? On what do you base that claim?
Let's be clear there is a difference between a strategic release of info (only part of a letter from the Zodiac killer, say) and a concerted campaign of false accusations intended to poison the juror pool before a trial even begins.
As you say, the "Foxy Knoxy" nickname was available on Knox's myspace page, so obviously that isn't relevant to a discussion of leaks by the Perugia prosecution.