The popup has to explain WHY it is asking for an email address. Else, Average Joe is just going to assume you are a spam site asking for his email.
I'm not sure I can do better than that text -- do you have any suggestions?
Edit: Sorry, on review this post was tangential to the point to which you were responding (about why the email is needed). It was targeted more at the point about user confusion by the login process.
Also, the "Learn More" blue text disappears on a gray background. That part needs to pop.