'dated', 'medieval' - that a principle is old doesn't mean it's invalid.  Don't murder is a very old principle too...

The law _cannot_ move away from a human-centric viewpoint.  Only humans are _capable_ of obeying the law.  Only humans are capable of being responsible under the law.

The law is all about people - about how we interact with eachother - about acting in a civilized manner and what happens when we fail to act in a civilized manner.  This is _why_ we have laws about treatment of animals - not because animals have any rights under the law, but because people who abuse animals are not civilized.