Part of it (not all, by all means) is that when neighbourhoods are evacuated, the emergency personnel tell the residents anything that will get them to leave. They'll tell people "sure, you'll be allowed to come back to get your cats" if that is what it takes to get that person out of there. If the person acts like they're hesitating, they scream at them and threaten to arrest them. If the person asks them to make sure their pets get out, they'll promise anything to get that person out of danger.
I can understand that--from the point of view of the police or sheriff's deputies, it is an emergency situation and whatever they have to do to get people out of immediate danger, they do.
The police or deputies are going from door to door, yelling at people to get out NOWNOWNOW! People get flustered because even though they usually knew there was a risk, they were still hoping it wouldn't happen. Most of those people have never had any significant contact with law enforcement in their lives and most people are heavily conditioned to obey authority (see the Milgram experiments).
It's a tough, scary situation to be in. I honestly believe that until someone is placed in that situation, they really and truly do not know how they will react. No pet owner wants to think they would leave their pets behind! But until they're in the situation, they don't understand all the forces at work. It's scary, chaotic and confusing.