the box turning green feels as having selected something, not actually submitted, and the "Subscribed!" line is far from the where you did the action, and disappears after a moment.
I don't know if this is a good design or not, just sharing my opinion as someone else who took a bit to understand I had actually subscribed.
I also was sort of expecting the example to change :/
I had the exact same feeling - I was unsure of whether I should be seeing a "save" button or not - and I didn't know whether the site had saved my response automatically.
It's a very subtle approach, but it's a little too subtle for my taste.
It felt pretty obvious to me.