I don't have a big fancy privacy policy here, so I'm just going to throw this out there in plain English.
The email address is there so I can ping you, and I promise I only do this twice. Once in the preseason, and once right before the playoffs, to remind you that your picks were all shit. Even then, I'll probably forget or screw it up somehow, so there might not be any emails at all, ever. I'm not going to give that shit out, because I hate getting mail-bombed as much as you do.
I can understand if you'd rather not give that out, so there's a handy other field to put in a regular, non-email address, username. That's your login name, and it is required.
You need a password, too, since otherwise anyone who knows your login could swap all your picks to be even dumber than whatever you put in.