(US Constitution Article 1 Section 8 Clause 7) > The Congress shall have Power... To establish Post Offices and post Roads.
I should mention that USPS does get tax money when other agencies use their services, but they’re paying for it just like anyone else does. It’s not subsidized, other than indirectly by their monopoly on letters.
The Post Office's job is to deliver mail sent to you, not to presort it by "validity" on your behalf and then only deliver the mail they think you'll want... if advertisers are sending you junk mail and it's getting to you then the Post Office is doing its job.
>Why should junk mail literally nobody wants be subsidized?
Again, it's not the fault of the Post Office that junk mail is a business, or that no one wants to send you anything but that. As far as it's concerned, mail is mail, and the junk treated like any other correspondence. Junk mail isn't subsidized, "mail" is subsidized - that so much of that mail is junk is not the fault of the USPS.