Fact of the matter is that true altruism and corporations simply can't mix. What I mean is that money spent by corporations on charity needs to provide a return on investment. Whether by means of improving a brand's image (and thereby increasing brand loyalty and sales), or by stifling criticism (and thereby decreasing negative coverage or legislation and consequently sustaining sales), or by leveraging the association to sell products (and of course sales), corporate dollars can't be squandered.
For McDonald's I'd bet Ronald McDonald houses achieve all three with the photos here illustrating their use of Ronald McDonald houses to market directly to children.