Your ISP has to pay for extra bandwidth licensing to accomodate the extra traffic, and spam can become a major problem as your name gets sold or passed from one spammer to another and your bandwidth space is taken up with countless ads.
Spammers cost the basic Internet subscriber.
I don't have to pay the postage for junk snail mail. A few unscrupulous individuals actually use the most common products advertised to bombard the e-mailboxes of people that they don't like by registering these individuals with sites that collect addresses for their adverts. That is also detestable. If they use flyers or other forms of advertising, they foot the bill for their own advertising, just like the rest of us. Maybe Some things Should be regulated.