It was enabled on my phone - which I discovered at the dentist's office. Now, it's possible that I idly clicked "ok" while setting up my Pixel 2 - it's not out of the realm of possibility - but I'm not very likely to turn that on at all.
Germany has much more stringent privacy regulations than the US does - maybe that's the difference.
I've just recently set up a Pixel 2 XL straight from factory a few times over the last couple weeks, all in the US. Every time I had to opt in to the background music detection feature, even in the US.
I must have toggled it on, then.
Can verify that in the US it was off by default and I had to turn it on. It is a feature I like and use pretty often.