Are you talking about a film in particular or just generally about films, because I can assure you that animals have been hurt, maimed and even killed in movies, as have actors. I believe Cecil B. DeMille was rather famous for having the scenery and stunts as authentic as possble, which caused problems for animals and actors alike.
I think that the OP was meaning that people get more upset at a plot which involves animal suffering than a plot which involves human suffering.
I suspect that in part this may be because we are afraid the animal might actually be getting hurt - not everyone respects animals! But even just the idea of it will hurt us because it is never the animals' fault they are there, and so they should not suffer. Adults usually have a choice, although I get pretty upset at films which depicts people with no choice and who haven't really done anything wrong getting a lousy time of it too. And of course violence towarRAB children makes us so upset that it's rarely depicted in film, although may be hinted at.
I guess I'm just agreeing with the point that it has to do with their innocence and choice really.