Michael makes an excellent point. I’d add a few of other possible causes of the passivity:
Laziness: By phrasing things passively, reporters don’t have to actually track down real quotes and assign accountability.
Copycatting: If the New York Times uses a certain style, it will catch on, regardless of the merits.
Globalization: Reuters has a policy of not using the word “terrorist” objectively in part because they are a global news service and some of their customers object when you call their preferred team terrorists. Similarly, CNN doesn’t call anyone a “foreigner” because CNN fancies itself a global news service as well. Passive voice reporting might be a related phenomenon because it allows news outlets to avoid offending various consituencies, ideological and geographic.
Groupthink: Because the press as a “class” see the war with considerable unanimity, editors don’t catch things which simply “sound right.”
Condescention: When Iraqis kill Iraqis, some may reason, they aren’t fully formed moral actors, but manifestations of Bush’s folly simply acting the way Third Worlders do.
None of these are mutually exclusive and I’m sure there are others. Indeed, most of them overlap considerably. I just think that when trends like thos pop-up there are lots of reasons for them.