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Those answer explain HOW they can afford to put them online - not why they should.

The answer is Brand Recognition. MIT isn't competing with the places people would have gone to if they couldn't read this stuff online - it's competing with Stanford, Harvard, Caltech, Dartmouth. It could outbid them for better teachers, it could hire famous noble prize winners, it could publish important scientific papers.

Or it could associate MIT with computer science lectures in the minds of potential students, future hirers of future students and the world in general.

Oxford published the "Oxford English Dictionary" more than a century ago - rather than keeping the secrets of what words meant for it's students - and 150 years later it's still paying off in selling English language courses in China.