I wouldn't worry too much about people distilling off the alcohol, there's a bit more work involved than just putting the brew into the still and starting it up. You would have to do a few conversions first, otherwise you'll end up distilling off most of the GHB/GBL with the ethanol/water. GHB isn't hard to make, this method is just something to make it easier for those that want to use it.
Most of the reports from using this method have shown that to get what would be considered a 'light' dose of GHB would require anywhere from 10-20 standard drinks. Which when you think about it, 10 glasses of a bad cider vs 10 glasses of great homebrew...well you can work that out