OK, fine. (But it might be wise to read your homeowner's policy to see what obligations your insurance company has if you do something like that intentionally. If I were an insurance company, I'd put a clause in there saying I wouldn't be responsible for paying such a claim. But that's just me.)
Were they committing a CRIME against me when they tripped and got hurt? Don't forget, tresspassing and stealing MY property is still a crime. This isn't a booby trap, it is mother nature.
You can get seriously cut on a bramble if you climb over my fence with the intent to burglarize my house. The liability is exactly the same.
I already called my lawyer, and he said it was a great idea.