A while ago, because of a web big-wig being impersonated here on wordpress.com, wordpress made a change. If someone is trying to comment with an email address that is either tied to a wordpress.com account, or is tied to a gravatar account, they are going to have to log in to prove that email address is actually theirs. Impersonation on the web has been becoming more of an issue and that is part of the reason you are seeing virtually all places that allow comments requiring that you log in somehow or register with their site.
You can at settings > discussion disable the requirement for a name and email address, but those fields will still appear on the comment form and if they enter an email address associated with gravatar.com or an account here, they will STILL have to log in.
You can tell everyone, perhaps with a canned paragraph at the bottom of your posts to NOT enter an email address or name if you wish (after making the change at settings > discussion). Of course that means telling who said what will be difficult.
I'm afraid with the growing number of gutter dwellers on the web, things like this are going to be the norm from now on.