Two points:
1) Berg's father mournfully says that his son was a staunch Bush supporter and war supporter. Not your typical ivy moonbat, and not a human shield, and not working against the US (at least not intentionally). We don't know, so he may have been stupid and doing things he shouldn't have been, but it seems he'd be the type to hand out at FR, not DU.
2) I don't think it's a conspiracy theory to say that the story, as it's been reported, is very odd and has lots of gaps and unexplained parts. Why he stayed after he was done, why he was arrested, whether it was by the US or Iraqi police (under US control, at least nominally), what he did between the time he told his parents he'd gotten out and was coming straight home and the time he was kidnapped, etc. Sure, it's even possible he was undercover something-or-other, but that might be stretching it a bit.