If it's indefensible now why has it not always been so? Someone must have though it was justified in the beginning.
As for the cost of fuel for cars, it's also gone up for boats. The differential may have even got worse for boats, yet some cars are now getting 60+ mpg and your avg mobo only does 1-2 mpg.
And why should we pay road prices when we are not using the roads, and do not have the advantage of thousands of fill up places such as Tesco’s