@nanoha If the posts are seen publicly, which they are (and if they aren't, I would argue they shouldn't federate anyway), then disallowing subscription would be illogical (because I can just scrape your profile page).
And, most importantly: in a _social_ network, you should be able to get to know the other person's feed before you subscribe or verify a subscription. This is logically impossible if you have to subscribe to them before reading their posts...