I'm not quite sure what you're getting at about deterrent? I know here in NYC that UPS just parks as they please and has some kind of deal with the city to pay the tickets with a bulk discount
Their point is that for an individual, yes, it's not that much of a deterrent - but that individual can only park poorly with 1 car at a time.
The USPS has a fleet of many (hundreds of thousands?) vehicles, so their capacity to ruin it for everyone is much larger - but their potential liability from fines is too. So they treat it very seriously.
The poster is saying that spam is more like the USPS situation, where single entities control thousands of potential infractions, not the rich individual.
