A Brazilian CA trusted only by Microsoft has issued a certificate for google.com

Speculative guess, but it sounds like intentional collusion/coercion between government and big corporations.

ie: Brazilian government demands Microsoft to grant them MITM access from Windows machines, in order for the right to do business in the country.

That seems very unlikely to me.

Governments are usually sneaky with their evil plans. It is simply too hard to get away with something like that to make it a viable. Case in point, the fact you are reading about it on hn.