My guess is that they're running it free as a sort of massive beta test, once they've got the format sorted they'll start including paying courses - quite possibly as follow up to free ones (for example Scala I is free, Scala II costs you money).
And, personally, I'm very grateful to them for hosting such excellent free courses at the moment - the quality has generally been pretty high on the ones I've done, and the format is excellent. If they start making money off it, good for them.
How would that work online? Cheating is already a problem - there's nothing to stop you googling for the answer to a given assignment, or getting your clever best friend to help you out.
I wonder if it's part of a strategy of creating a huge userbase and alot of content and data all of which will help you lead the field in the future(and be the most profitable). And nothing helps more to create this sorts of things than "free".
And, personally, I'm very grateful to them for hosting such excellent free courses at the moment - the quality has generally been pretty high on the ones I've done, and the format is excellent. If they start making money off it, good for them.