At least there's some rationale for this. National postal agencies typically have monopolies, but the flip side is they have universal service obligations, i.e. they have to deliver mail to even unprofitable locations, e.g. rural areas.

This is analogous to taxi regulation: in theory, the monopoly granted to medallion holders has a flip side: rates are fixed, and they can't discriminate between riders based on destination.

In practice, taxis in NYC skirt these regulations pretty often.