Basically every major subscription service is trying to expand its offerings to make its subscriptions stickier. In this case, you might want to cancel one of Netflix or Disney or HBO for the next couple months, and you were going to cancel Netflix except your kid is loving World of Goo, so you'll cancel HBO instead.
Sure, Netflix could license World of Goo and charge $5 for it directly. But not that many people are going to buy it. But literally 100x more people might download it and play it when it's free with their existing subscription, and then if just 1% of them extend their Netflix account for a couple months because of it, it's much more profitable for Netflix.
(Edit: I don't work for Netflix, this is just general industry strategy right now that I'm describing.)