There is extensive case law on possible criminal responsibility for a property owner who sets a trap for trespassers that causes personal injury. It looks like this is just a little jolt, but considering he lives in Washington State he should probably stop this.
I would guess that it might be a problem in a city or burb where the homes are all close together and not all fenced-I live in a remote rural area and trespassing and/or theft on private property is taken very seriously here. Everyone has fenced property with locked gates across the driveway-and a lot of people have those gates and fences wired with non-lethal electric.
No judge or sheriff has any sympathy for someone who ignores a no trespassing sign on fence or gate and gets shocked-that is what they get for not respecting private property-they are lucky-getting arrested or shot would have been worse...
Rub the signs with Poison Ivy. Identify the culprits by their Calamine Lotion covered hands and arms!