Sometimes people just want to follow your updates on Facebook, rather than get your information from two different sources. If you're really proud of your email newsletter, you can add a signup form to your Facebook page to encourage your followers to also sign up for your newsletter.
Going to your followers' profiles to find their email addresses and adding those email addresses to your list is not allowed within MailChimp. This is considered "harvesting" or "scraping" emails. See more lists that are okay or Not Okay to use in MailChimp.