I think you're misunderstanding. The AC above (not me, as you implied) stated...
"MS will be able to claim tax benefits for the "donations" based on the retail price of the OS. In other words, they'll be making money from taxpayers themselves"
And, you clearly disagree. I read his statement as saying that if MS would have paid X dollars in taxes on their income, and now is paying Y, because they made a charitable donation, which really didn't cost them anything but the licensing, then they've successfully avoided taxes they would have normally paid, and in a rather sleazy (but legal) way. So, the difference between X & Y is lost to the government, and thus all of us, because it would have gone to services we all pay for, and now it just piles onto the national debt.
Now, you've brought another issue into the discussion, that you believe (as do I) that the government is already spending more than they should. I in no way disagree with you on that, but it is secondary to the points above.