I is cheaper for Amazon to fork Android if they want their own thing. Think about it, they already have their expertise in Android.
As for the tax thing. If you willingly give money to an institution, it is called a donation, and people donate money all the time. I don't understand this confusion between a donation, and a tax law. You should not be allowed to opt out of tax money that pays for the infrastructure that made your business successful (Internet, roads, laws, police, fire department, investment in basic research, railroads, education schools and universities, regulations)
The flip-side of the argument is, if you don't like to pay for this ready-built infrastructure (both physical and cultural), you should move somewhere where such infrastructure does not exists, therefore you won't have to maintain it (insert your favorite third world country here) instead of insisting on destroying the infrastructures that we have here.