Plenty of companies pay the post office to deliver paper spam to my mailbox each day. I don't want any of that, either. "Free" makes spam easier, but a cost doesn't prevent it.
The post office _wants_ some spam ads in the mail. They get paid less but everyone gets it. At this time without mail ads there would not be enough mail most days to send a postal carrier out. Think of it as the fiber in your diet. You don't get anything nutritional out of it but it helps move things along.