Just asking for a few pennies to take part would up the completion rate and reduce the frivolous sign ups.
1 - Sign ups alone require nearly no resources from the provider.
2 - Attendance and therefore popularity could be much lower. While that could be a good thing (less people and more money -> more support to each student), it would probably never be enough to cover a course's cost.
In the end, people are learning. I haven't completed any course yet (attended about 4, attending one now), but I'm thankful for them and I don't think the resources I used harmed the MOOCs.