That's the thing, though; some websites will let me pay them directly for an ad-free experience. The post office won't.
I would be happy to pay the post office $5 a month just to say "resident doesn't live here" I'm always afraid a check or bill or something will get tangled up in the 'penny saver' - hell, you could probably get me to pay more for more sophisticated filters, but even $5/month has got to be a hell of a lot more than what the bulk mailers are paying to get into my inbox.
And it's probably an even bigger win; I'm not the sort of person who reads the penny saver; the post office is making more money /and/ saving the penny saver people money if they let me pay them not to deliver said periodical.
The fremium "Pay me money, or watch ads and the advertisers will pay me, your choice" model is an old one in internet terms, and one that most people seem to be pretty happy with.