I'm sorry to hear this...

I don't know what your laws are like there, but here in the US, it doesn't matter if you have signs all over. If a dog bites someone, no matter where, they are legally responsible. Even burglars trying to rob a house and who got bit by the family's dog was able to sue the owners before

It is totally not right IMO, totally messed up. Hopefully the laws where you are, are more reasonable.

If that man had just remained calm instead of lashing out, he'd probably be just fine.