The fact that this is a forum, and not a mailing list, is easily confused, since you don't have the ability to subscribe to a single thread, but only to the entire forum. Thus, if you want to make sure you get notified about something, you have to receive emails about every single post here. Considering this, and the high rate at which most people make accounts on web sites, it is perfectly reasonable to only require an email address to retrieve login information, so as to be able to stop the emails. This is, in fact, used quite frequently across the web; if you don't have access to the email account, then nothing happens
You will also note that the OP was having difficulties getting logged-in, without which he could not unsubscribe. It was this that was the meat of his complaint, rather than the ability to unsubscribe once it was available to him.
The problem mostly lies in the fact that this is neither a forum nor a mailing list, but a Moodle-hack. I have suggested using PunBB, but the argument against was that the software needs to have good capabilities for displaying math. I haven't actually seen anyone post any equations here, but...