It’s a user experience best-practice to make notifications as easy as possible to adjust. This becomes imperative if these are created by default during a reply or submission. Additionally, an email should contain instructions on how to disable or turn off notifications in the email body. There is no explicit mention of enabling or disabling email alerts anywhere on the site that I have seen so far.
Because there is no explicit opt-in to receive alerts past the initial transaction of submitting or replying, these emails may be in violation of the CAN-SPAM Act of 2003.