I think if you work 8 hours then you bill 8 hours. If you work 6 hours, and go home, then you bill 6 hours. If the job is 2 hours drive each way from the office and you have to pass the shop to go home, and you work 4 billing hours, then the 4 hours traveled get billed to whomever the customer is.
I have no question that billing for 8 hours and working 6 equals theft of what ever 2 hours labor costs, from whomever it is billed to,
I also have no question that it is theft if 2 customers get billed for for the exact same techs exact same time frame, so you can not bill 2 different customers for the same travel time, even if your billing is "portal to portal" but instead of returning to your shop you go on to the next customer, only 1 customer can be expected to pay for that time.
Just because time is an "intangible item" does not mean that it can be expected to be somehow different than a tangible item, as far as how it is sold. (you can only sell 1 lamp to 1 customer, and you can only sell that 1 time block to 1 customer)