While the statement could have been worded better, there are things to consider, and that's liability of the sponsors/coordinators: WFPF, Redbull and of course MTV, home of Jackass. Also, is there anyone out there that hears: "do not try at home" and then decides to never attempt that sport or activity, and then goes through litigation to have the activity made illegal to prevent others from trying at home?
Since everyone seems to be so lawyer happy these days, there has to be some modicum of a disclaimer. Besides, starting in your backyard might not be best for every practitioner. If it is a WFPF event, they would have information on getting started, websites, what a suitable training location would be, any special facilities for PK training like Tempest's Academy, etc. And if not, then the people running WFPF don't care about the sport, if they cant xerox a few info packets but can organize massive multi-day events that cost thousands of dollars.
For the TL;DR: People driving out to these massive events are probably going to ignore the "do not try at home" statement, or at the very least, look into the sport before running face first into the edge of a picnic table.