I think the point was that many on-site employees get their work done just fine, but spend half of their in-office hours goofing off on Facebook or reading HN or whatever. So in principle that "wasted" time (assuming it is indeed a waste) could be put to use for a different company, with double the compensation. Obviously there are severe logistical issues if you're on-site!

True but compared to the number of people who are just unproductive at their single job, there’s no point in worrying about people working 2 jobs.