Charitable donations don't benefit the giver at a 1:1 ratio. MS would be better off investing $20 million and getting a 10% return on it than taking a gamble and purchasing a bunch of computers for underprivileged schools (hint: kids that go to these schools most likely aren't going to be buying computers anytime soon....)
I don't see how you can say that with any degree of accuracy. In one respect, making charitable contributions is like advertising. When you advertise you try to do it in such a way that it entices people to purchase your product, or your services. Likewise, in a manner of speaking, making a charitable contribution is very good for your company's public image because it gives people a "warm and fuzzy" and people naturally want to buy your product, or your service. As an added bonus, it will hopefully generate future sales. Now initially, when the computer donation was made, I would agree completely that the tax benefits would not cover the cost of the donation. It would be silly to think that it would. But how do you measure the worth of the good pulic relations associated with the gift, and how do you measure what future sales will be because of the computer donation? As with any type of advertising, there is no real way to predict that.
As far as kids buying computers, I would suspect they will not be purchasing a computer of their own until they at least finish high school. But when they do purchase one, most likely they will purchase something they are used to, which of course will have a Windows operating system and other Microsoft software.
Once more let me remind you that I am not saying people, or companies, make charitable donations simply for the tax breaks, or for the improved public image, or whatever else that may be involved. I am only saying that making a charitable contribution does have benefits for the giver, and in some cases the benefits may be substantial.
I would also like to mention that I wish individuals and business owners would make charitable contributions more than what they do. I think this country would be much better off if everyone were "more giving" than what they are. I think collectively we are much too greedy and excessive greed will ultimately be our downfall. A little greed is OK, excessive greed is not OK.