Suicide is illegal because of a legal techncality, In order for it to be illegal to help someone kill themselves you must first make it illegal to kill yourself. They can't make it illegal for someone to help someone do something legal.
The fireman are not allowed to let someone they've rescued back into the house, They are legally responsible for there safety. It would also be incredibly foolish for the man to have tried to go into the house unprotected as an untrained civilian. Even a small fire can cause enough smoke to make him pass out in moments or even cause structural damage to the house. Theres a reason Firefighting PPE gear wieghs over 50lbs.
The specific reason for the firefighters not entering the house to save the cats in this particular case is unclear, but the a decision like that would only be made if it was deemed too greater risk for the firefighters lives, the fact they didn't go in also means the fire was probably quite large by this point. As someone already said, as cruel as it sounds, they're just cats.