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.