Oh I see, 'subscriptions' as a counterpart to 'circles'. That's not a bad idea, it would also translate pretty well to Youtube channels. I guess they didn't introduce it since it would confuse many users 'what's the difference between circles and channels?'. They could have made it as an optional feature for publishers, i.e. only shown to users who enable it in preferences.

edit: Users who subscribe to a feed would always see the feature if the publisher has enabled it of course