I like how the parents were ninja to the end about this whole thing. They knew what it took for Itachi to make that decision and realized the conviction that he had, and respected that. It wasn't the "oh how could you?!" reaction I had in mind.
It's entirely plausible that Itachi had them in a genjutsu that prevented them from moving. With everything we've seen in this manga, and everything we've seen Itachi in particular do, I think they were just fully defeated at that point, as opposed to still being able to fight back but just giving up.
Also, the brotherly love thing was beautiful. Beautiful I tell you! *single manly tear*