I thought it was more to do with Qualcomm's SOCs having integrated radios for the atypical US bands, something they'd have to add on otherwise?
For instance, the only band an S20 is missing that's commonly found in the US is 66.
https://www.deviceranks.com/en/phone/bands/16582/samsung-gal...
These days it's purely a patent licensing agreement. Samsung doesn't sell Exynos modems (and thus Exynos SoCs) in the US or China (historically markets with lots of CDMA), and Qualcomm doesn't hassle them about the rest of the world.