Is that really a flaw? If I can learn better by reading books and then working through problem sets, why bother with the lectures, especially if I am strapped for time as many MOOC participants probably are. I am a huge fan of MOOC's, I've probably attempted about a dozen and have completed about half. So far the biggest reason for failing to complete a class has been time constraints. If I am already familiar with the material (i.e taking the class as a refresher) or don't find the lectures particularly engaging, I will still 'watch' the lectures, but play them in the background while I work on problem sets or do other stuff. In my experience, most MOOCs have a healthy portion of participants who are very familiar with the material, so it's quite possible a good percentage of them are just using the problem sets as a way to refresh their understanding.
Unless of course, you are insinuating that a majority of these people are cheating (i.e. copying and pasting answers on HW's) I don't really see what the big deal is.