no, it's because European users sh** eaters, almost every android phone has limitations in firmware in the European market (lower volume by 10%, or haven't eq, like my old OP7)It is possible that this feature may only come to countries with strong anti monopoly laws. Russia has asked phones sold there to offer local Russian alternatives to set as defaults, and without the ability to customize the button, Sony may have been deemed anti competitive. So they added this option to make sure they don't get fined/kicked out of the Russian market. I assume EU countries may get the same option.
The rest of the world? Ask your government to fine Google for anti competition and maybe we will get that button to be customizable too.
I'm only basing myself on assumptions, but the EU has already fined Google multiple times. Android users in the EU already have a ballot system where they are allowed to pick what browser and what search provider to be installed and used during setup in phones officially sold in the EU with EU region firmware.
While I agree that Sony is getting from bad to to worse in terms of supporting their own phone's Android, I believe this is purely a marketing decision, either by Google or by Sony, or both.