This only relates to Post Office Boxes in post offices.
When I buy a mailbox at Home Depot and plant it at the end of my driveway; that is MY personal property; not my governments.
Not that many decades ago, most people in small towns got their mail at their post office in their own post office box. I can remember ours from about 1960 or so.
If the PO owns your mailbox ask them to replace it when some kid blows it up.
Not quite. The PO will not replace a damaged box or remove it if it is no longer in use. However it is protected from vandalism and from use by other than PO employees.
The government considers mailboxes their personal receptacle to be stuffed with whatever they deem appropriate. That is, any material bearing a PO stamp. All other material is contraband and its purveyors are subject to penalty.
Deliberate destruction of mailboxes is similarly subject to penalty. Removal of your personal box is a gray area. There is, as yet, no law requiring a citizen to receive material from the PO.
This is a federal, not a local issue. Hence, the stickers are a violation of federal law unless placed there by a PO employee or their representative. Proceed with extreme care.