When I first had the car, I found this to be totally ridiculous. Whoever heard of a car that automatically turns on the radio when it starts? But, after having had the car for a month, the problem is already fading, as I've developed a habit of turning the volume down before I turn off the car. At this rate, six months from now, like you, I'll be defending this anti-feature. (Hey, you're going to turn the radio on anyway when you drive, so why not have the car do it for you on startup?)
I agree that the key word is rental, but my conclusion is exactly the opposite of yours. The fact that this is an unfamiliar rental is what is allowing the OP to see the issues with the car's UX clearly. If the car were his daily driver, he'd have adapted to it, and he wouldn't notice himself getting distracted.
[1]: https://www.subaruxvforum.com/forum/interior-audio-hvac/1460...