I must admit I only read part of the WSJ article, but I think the idea is to get people to submit voluntarily, much the way people have voluntarily surrendered their privacy via social media sites. Of course, at some point the question becomes, "Did the person submit to the procedure voluntarily, or was s/he persuaded into believing the decision was voluntary?"
I know nothing about paintball, but I've been told the "paint" is really soap, though it can still stain clothing. I think it would be better to use BB's or pellets from a CO2-powered BB gun since those can possibly puncture the tin foil panels on most newer cars, leaving small holes, and if it doesn't puncture, it will leave noticeable dents.
This reminds me of the Gallagher joke about all the stupid drivers out there and how we should be able to shoot suction-cup-tipped darts with "Stupid" flags at cars, then, when the driver accumulates 10 darts, the police can pull the person over and write a ticket for being stupid.