FLVS courses have, or at least had a few years ago, live teachers who monitor student progress and are available by e-mail during large portions of the day and by phone during their specified office hours, and who have phone contact with the child on a regular basis, so they are already paying a teacher. It wouldn't really be more resource intensive to throw up a discussion board for each section and make it mandatory for students to participate (which I think would improve most of the courses greatly), but it would require a different kind of work for the teacher - it would require being less evaluative and more engaged, and require thinking about the curriculum in a different way - and I think that that may be the crucial point in why it doesn't happen more. Sadly, a lot of teachers really do think of education as filling bucket rather than lighting a fire.
