The courses are free, that's the trick. I'm not paying a penny, for a really high-quality course.
And before you ask, they ask for money when you want to be accredited for the course. They have some kind of signature scheme that establishes it's you doing the homework and exams, and then you get a certificate at the end.
For the course I'm taking, 30,000 people have signed up. But, statistics say that about 5% actually finish it to the end.