A better way to sell natural gas would be by the energy content, so much per kilocalorie (a term directly translatable to BTU’s, which nobody understands either). Compressed natural gas does not come in “gallons” or “liters” but for a given quantity of fuel by weight, contains a given number of BTU’s or kilocalories.
And this is what provides the total energy needed to propel a vehicle down the road.
BTU is not a metric unit, but a clumsy engineer’s archaicism. Nor is a kilocalorie a true metric unit, but a metric-derived unit which has the additional disadvantage that there is nomenclatural confusing over the term.
The metric unit of energy is the Joule. To use the Joule in commerce would be a problem, because all the mechanisms for dispensing the fuel would have to sense the energy content, which surely is impractical. It is better simply to adjust the price according to the energy content of the fuel, just as we have different prices for Regular and High Test gasoline.
I think your point is reasoned. I realize the vapor vs fluid part of it does present a need to reference it differently.