Posters have the option “When someone comments on your post” in “Email settings”, but apparently it’s not active by default, which could lead to missing messages. Also there doesn’t seem to be an option to get notified for new comments on a post you commented on.
To comment on your first point, with the new European GDPR coming up, stuff like this will only be opt-in by choice. It will no longer be allowed to have options like this on without the users consent.
It could be somewhere in the stuff you agree with when you create an account. Isn’t [hat a common practice?
That’s what they would call a bundled opt-in. Those are no longer allowed under the new GDPR. Users need to approve any type of usage of their private data one by one. Which in my opinion, wouldn’t be a very pleasant experience if they had to check 15 checkboxes before registering an account.
That is why most companies have chosen to just turn all of these features off by default and give users the choice to opt-in later.