When governments are not a huge customer of Microsoft, there might be some ground to complain about them being subject to anti-trust laws.
For the moment, "Microsoft tax" is far too literal. And your comment far too close to the usual silliness of reducing regulations on government-supported monopolies...
I'm having trouble with that statement.
The Government chooses MS based upon their criteria and needs - whatever they may be - and they would know exactly how MS does business because they investigated the products and licenses - right?. Now, after choosing MS, they decide after the fact that they don't want to do business anymore based on the terms they implicitly and explicitly agreed to so, via the law, they change the agreement.
Basically, this sets up a precedent where a company in good faith makes a deal with a Government, and then the Government throws everything out the window and they decide what you will take - maybe give them nothing.
That's scarier than any Microsoft's business practices.
Oh, and they do have an alternative - just ask Richard Stallman.