Instead, they damaged it and stole part of it.
You can't steal what is given away, and the law says that if you leave stuff lying around on purpose, that you're in a grey area. Specifically, by leaving it on the side of the road without any sort of permit, you're littering. Not only are you giving up your rights to whatever you left there, but you can be fined for discarding of waste improperly.
Now, I agree that it's douchey to damage the thing and take part of it, but I don't agree that it's vandalism or theft. Some social experiments fail. Or, alternately, it's a success in another way; it's taught us something about the world.