Have you tried changing your location in your Sonos account to a different country where Amazon Music is supported?

Amazon is the controlling factor here. They need to authorize Sonos to carry their content in any particular location, it isn’t up to Sonos to make that call on their own. 

Do not try to blame Amazon for a lack of Sonos action to add more countries support.
Why Sonos does not enable other countries where Amazon Music is available?
 

Because lawyers and copyright laws. Amazon’s content is licensed from musicians by Amazon. It is up to those legal agreements to determine exactly which countries are supported, and how. Sonos cannot simply play content to users anywhere in the world.

So, what stops you to add support for other countries where Amazon operates?
I am not talking to allow do something illegal.
You had to taken approval to support currently supported regions. I do not believe that Amazon for some reasons blocks you to add supported by them markets.