Interesting. At least in Norway its quite common in my experience for a driver to turn off the meter if they've made a wrong turn or similar obvious mistake that makes the trip go long, were the driver is clearly at fault.

I guess that means that the only way for a driver to give discounts, would be to pay the difference themselves, incurring tax on the discount?

They're a blue collar guy trying to feed their family on a low-wage job with variable income, they have like zero incentive to do so man. They're also gambling that they can potentially intimidate a rider who says they won't pay x price because of a detour, mistake, or misquoted price. The driver could be larger or tougher than the rider, and you're in a small space with them.

I had a larger driver try to intimidate me into giving them a cash tip on top of the one I'd already written in via credit card. (I refused). This is why I never, ever use taxis now