That one is haunting me too. It was totally unnecessary and the little boy is obviously way too young to philosophize about the relative value of humans and pets. He's human and he matters! Dealing with this little boy in the Astrodome would have been a whole lot easier for everybody if he could have taken his dog on the bus, and been assured at the other end that his dog was safe in a nearby shelter and that he could visit the dog. As it is, he's surely difficult and disruptive.
Even in emergencies like this, there are rescue workers that let things go to their heads. With that little boy, it was probably some jerk that got of on the power he had rather than the compassion he should have had.
More importantly, however, kiddo may miss Snowball, but what happens if you have 5 or 6 people on that bus who are badly allergic to dogs or cats? That's asking for a bad allergic reaction from people whose bodies are compromised as it stands already.
It's sad that the kid had to lose the pet he loved - but, in a disaster, its what you have to do. It sucks - and I feel sorry for the guy who had to enforce that rule - but it is absolutely necessary.