It seems to me that the fundamental difference between this and other platforms that require login is this: for Facebook, having an account is the product. Face

It seems to me that the fundamental difference between this and other platforms that require login is this: for Facebook, having an account is the product. Facebook has designed everything about the Facebook experience to funnel users from registration to "engagement," and this is their main line of business and revenue source. Anything Facebook does to require someone to have an account is also forcing them to be a customer of their principle product. The same situation doesn't really apply to Microsoft or Apple. No doubt there are examples of both trying to monetize account holders (e.g. cross-promotion of products) but neither of them views the "having an account experience" as being the product. You don't "sign up for Microsoft" in the sense that you sign up for Facebook, the account is not a feature but just a tool for providing identity to various products. This lets Microsoft and Apple make a far stronger argument that requiring an account to use a product is just a technical aspect of the working of the product. Facebook would find it extremely challenging to make this same argument. Google might find itself in a position that is more of an awkward in-between, except for basically all of their attempts at either generalized social media or meaningfully unifying the social aspects of their products have failed, so if they're trying to funnel Google Docs users into being YouTube users they're at least miserably bad at it.