This could all be handled by settings in the browser, only if the sites themselves listened to the users' browser preferences.
It makes a bit of sense, since the mailer had already paid, but the main justification (iirc; it was years ago that I read the opinion) was that a postal service should be neutral and trusted to deliver.
The companies on the ads wouldn't do it that way if they were not getting a positive ROI from it. They probably only need to get 2 maybe 3 new customers to offset the cost of mass mailings.
Should USPS be required to respect that owners wishes here?
Sensible decision I think.
The user agent should... be an agent for the user, and be able to perform actions on their behalf.
(The legality of those actions is of course assumed by the user here... if I add an automated flamethrower to my mailbox and burn my bills, well the debt collectors may come regardless if I read them or not - we cannot shift blame to the USPS here).
The second argument has the problem that for the whole thing to work the recipient must also have reason to trust the post office, but here their interests are not considered at all.
Ultimately there’s no good excuse for the banner solution.
Same sort of thing when you log into Wizzair and the check box below the password field is not "remember me" but "subscribe to our marketing emails".
If you build a website without all that tracking stuff and without 'free' services from the data collection companies Google and Facebook, then you have a pretty good chance of not requiring a banner at all, because for logins, etc., you are allowed to use cookies et al. without requiring an opt-in.
But I never saw anybody at the OMR being proud about the state of cookie banners they created...