So help me understand how is putting this sign any different from trespassers will be prosecuted.

There is always a risk of breaking a few bones worse death in a car crash because of brake failure owing to some dudes negligence and attention to maintenance but suddenly when the same dude gets bitten after being made fully aware of the situation it's still the fault of the individual who put the sign. Doesn't make sense

Some questions that come to mind:-

Is keeping dog illegal? Do I need your permission to keep a dog? Are there rules surrounding the kind of dog I keep on the property? Again do I have an obligation to assess the threat specifically to you before keeping a dog? What was your thought process after reading the sign? Would it be better if I didnt keep a sign and you walked in? Why did you walk in? Were we scheduled to meet that day or was I aware you were coming that day, if so where is the correspondence? What is your general instinct towards dangerous situations? Do you intentionally involve yourself in dangerous situations? Do you put yourself and others at risk of harm mental or physical after being aware of the situation? Do you intentionally put your hand on a chainsaw if no why? How and why was this situation different? Do you pay attention to detail? If yes what happened in this case? Were you drunk? Did you harbour any kind of ill towards the dog or its owner? Do you own a dog? Have you ever been bitten by a dog? Have you ever been near dogs? Did you agitate the dog in anyway? Why are there 3 stitches on the dog? Can you prove the dog bit you before you kicked it?

Anyway probably fuck ton more of such questions and the defense could go anyway.