It is not for the USPS to decide which mail gets delivered or not. They get paid to deliver mail, and they deliver it. This is a wild take on “the postal service should now do anti-spam”.
It is absolutely the job of the USPS to be a public service. Many countries' postal services have the concept of letting individual recipients opt out of bulk mail; for instance, in the Netherlands, you can put a "NEE" sticker on your mailbox. Or, better yet, entirely discontinue the specifically designed bulk mail programs that exist to deliver postal spam.
I'm not suggesting the USPS should be filtering by content, just taking the programs that exist specifically to facilitate postal spam and consigning them to the dustbin of history. Those programs essentially exist because they're a primary funding source for USPS.