It was ironically done for privacy. Users actual e-mails were shared with friends, and people had too many "friends", causing their e-mail to be leaked. After a bit of trial and error on hiding the e-mails from scraping, they eventually hid all e-mails by default and only exposed the @facebook.com emails. The original idea of people only having 'real' friends turned out to not work. You can't trust your Facebook "friends", which should be alarmingly clear for everyone by now.