> Interestingly, I think login walls are to stop spam.

I don't think that holds up in general, and especially not in Twitter's case. Logged-out users can barely interact with the site -- all of the important interactions (and, in particular, all of the ones that would be concerning from a spam-prevention standpoint) require the user to be logged in.

I'm not sure it applies to YouTube either. The site has very few login requirements, other than for age-restricted videos.