I pay for commercial mail by needing to constantly empty my overflowing mailbox and trying to make sure I don't lose any important mail in the process. And we, collectively, pay for spam mail because of the externalities on our society including having all that waste paper collected and recycled (and recycling isn't free or 100% effective).
Externalities are a cost we can't ignore - I'd rather pay tax dollars to support a good service than subsidize a shitty service with advertisements.