>There's no good reason it should do so when backgrounded.
Wrong, and it's especially irksome that just because you cannot think of a good reason, you leap to the unfounded conclusion that there is no good reason.
When you are driving and not using the app and not even thinking about the fact that you have the app, and not planning to use it, it can still be gathering useful traffic information to help route other users according to the current conditions on the route you are traveling. By the way this takes a super negligible amount of power because it can piggyback its use of system services (location, data transmission, etc.) on top of other already running services.
Then explain that to users and let them decide what they want to share instead of opting everyone in by default with no choice to opt out (this is really more "forcing" than "opting" if there is no option).
There is no opting everyone in by default for location on iOS. They have to explicitly opt themselves in, if they want to. Otherwise they are opted out by default. This goes for all apps and is enforced by Apple in order to maintain a good experience for the user.
I can't think of an example where it would gather useful location data while I'm sitting at my home that couldn't be inferred if it was only allowed to use geo while in-app.
Just because you can't come up with any examples doesn't mean that other people can't. You aren't the only person capable of thinking of examples.
Some examples:
It's useful to know when you have arrived home and are no longer moving.
It's useful to know when you have started moving and left home.
It's useful to know where your home is so that it can guide you home and for other traffic related reporting reasons listed above and below.
It's useful to know where other homes are or other places you spend time, where it may be significant that you are arriving and staying for a long time, as opposed to for just a minute or two (as at a traffic light).
It's useful to know that your non-motion is not an indication of traffic being stopped on your road.
It's useful to know that non-motion when you are away from your home may be an indication that traffic is stopped on the road you are on.
All of these are directly relevant to explicit, openly featured Waze use cases.
Wrong, and it's especially irksome that just because you cannot think of a good reason, you leap to the unfounded conclusion that there is no good reason.
When you are driving and not using the app and not even thinking about the fact that you have the app, and not planning to use it, it can still be gathering useful traffic information to help route other users according to the current conditions on the route you are traveling. By the way this takes a super negligible amount of power because it can piggyback its use of system services (location, data transmission, etc.) on top of other already running services.