By launching the attack in a full page ad, they initiated the battle. Had they done this critique (and many did) in an op-ed format, they wouldn't have invited
They weren't attacked because they said "Betray Us". They were attacked because they pulled a full page ad out and were questioning the President's plan on Iraq. The fact that they used "Betray Us" wasn't the issue, it was merely an opening for Republican to attack.
Their words and brow-beating about not attacking a military office had nothing to do it. If one of their own does it (say for charging people for doing their jobs - Abu Ghraib / Sniper baiting) they're let off the hook because, in the end, they don't want to attack their own.