Because the law is on their side, that's why. Information about the donors goes only to the Internal Revenue Service. That's a scandal. To be sure, the organizations receiving money usually insist donations don't influence their public positions. Among other things, it helps voters learn who's paying for campaigns, both for public offices and ballot propositions.
More disclosure is needed in all kinds of arenas.
The Microsoft case is not just about business. It's highly political. Here's a company that has engaged in thug-like tactics for years against its competitors and even its customers, as the antitrust trial record showed. It's a company that has already been caught planning a phony grass-roots campaign to boost public opinion. We only know some of the recipients.