It is sad because animals/pets in all their innocence have absolutely no understanding of what is happening.
I'm sure this little boy couldn't understand either. And in light of all the details, I don't understand. Rules shouldn't be allowed to trump common sense. Having that little doggie on the bus couldn't possibly have been more disruptive than having a hysterically crying and vomiting child on the bus. And at the end of the ride, there were animal rescue workers prepared to take pets to nearby shelters, so owners could be assured they were safe, and even visit them.