No, it isn't.
The issue isn’t telling others what they should or shouldn’t do, the issue is warning those who decide to trespass on your property.
A sign isn't needed to inform someone that they're trespassing.
Once someone has trespassed they've already broken the law, whether common or positive.
And to then do damage to a person's property? Do you see the problem with your assertion?
Is that in accordance with common law?
And as far as 'booby traps' go...
If you watch any of the following videos (https://www.google.com/#q=scary+surprise+video&tbm=vid) is it a 'trap' if I don't warn you and you have a heart attack? Or is it simply a prank as I only intended to scare you?
To take things even further could someone sue a movie maker when a 'surprise' scene comes up on a screen and they suffer from ill effects? How about a haunted house at Halloween?
The man intended to do harm to someone's property after he trespassed onto their property.
My opinion is that the pissing man got what he deserved.
He could have left after stomping the sign yet he didn't.
But that's JMO.
That is right. The purpose of the sign is to WARN the trespasser of a dangerous condition on the property.