Bloody hell, that is a real low point for the guys using those trails. I'm glad no one got their head chopped off.
I would love to catch someone in the act of planting these crappy devices. I'm pretty confident that they'd have a miserable time at the hands of the guys they were intending to hurt.
I think the intention isn't to stop a rider or a walker from hurting the environment, I reckon these braindead sponges are just out to get off on harming people. If caught, they will certainly do some time and get to know a bit of barbed wire real well............scumbags.