So while the mil isn't really used anywhere else that regular people see any more due to inflation, it is a valid division of the dollar and that's why they are able to get away with it.
No, it's purely stylistic. We tend to spell out denominations on coinage and "dime" is just the American spelling of disme, meaning a tenth.
The capped bust dime from 1809-1839 had "10 C." rather than "One Dime". Similarly, the capped bust quarter said "25 C." instead of the modern "Quarter Dollar", the half dollar said "50 C." rather than the later "Half Dollar" and the half dime said "5 C." rather than the later "Half Dime."
Most of the 18th century and early 19th century coinage, besides half pennies and pennies didn't have their denomination written on them at all.
When the final calculation of XX.YYY litres * AAA.9 pence/litre is done, it's then rounded off to 1 pence.
Currency conversions are also frequently done with readers that aren't a round multiple of pence, even in official government tables: https://www.trade-tariff.service.gov.uk/exchange_rates/view/...
They're allowed to get away with it because of a dysfunctional lobbying driven government. Mils don't exist in the common knowledge and if any reasonable person looked at this they'd call it out. It is useful in accounting but a Mill has never been minted and the last half penny was minted in 1857. It has never been possible using issued physical legal tender in the US to pay a debt of $3.129
The Mill doesn't exist because of some archaic need - it's pure dysfunction and the utilization of it in gas prices is a practice that should and very easily could be made illegal.
So I don't think it's just "evil retailer tricks".
