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.