Because (according to this announcement) if you don't, Google will put you in the spam folder.
Edit: I suppose it does say "unsubscribe from commercial email in one click". But it's hard to say exactly what they mean. They also don't define Bulk Senders - is that the domain or the sending SMTP server?
Edit: I suppose it does say "unsubscribe from commercial email in one click". But it's hard to say exactly what they mean. They also don't define Bulk Senders - is that the domain or the sending SMTP server?