Why are users advised not to opt in to mailing lists online?

Opt in emails can actually be a very good thing to receive. They often contain special promotions and deals only offered to those who chose to receive the email. Opt in mailings are also used by spammers but those are not as easy to handle. Many times the email will say that by not responding you are opting in to receive the subsequent emails. That, however, is not quite the same as opting in for an email.