This is intersting. From a UX perspective, you're basically forcing the user into making a choice on the spot: to subscribe or not. You're in or you're out. It takes away the non-commital mind wandering, nudges those who are favorably on the fence to sign up ("why not; I might forget about this site so let's do it now") and weans out those who don't immediately see the value.
Exactly. It’s also kind of jarring in a world where content is unlimited, free, and available on every channel. It piques some people’s curiosity to land on a site where it’s semi-restricted.
Exactly. And if somebody subscribes, you have the chance to reach (and thereby help) them many more times in the future than a fairweather fan who forgets you as quickly as their feeds refresh.
For this use-case, it's probably a good thing.