At the risk of being cynical, I think that's the point. If everyone who could contribute considered them (corporate sponsored hackathons) as work for free they'

At the risk of being cynical, I think that's the point. If everyone who could contribute considered them (corporate sponsored hackathons) as work for free they'd quickly run out of submissions and fail. Keeping the perception as a way to excite and/or challenge developers is pretty well essential for their continuation.