Apple isn't the only company that does product placement, but they are by far the most common computer company that does it. Most others rarely, if ever, do it. The only recent example I can think of for another one is Dell in V for Vendetta.
Well, first of all, MS and Dell both have major presences in a lot of movies and TV, big and small, from Marvel through the new Tomorrow People. Second Apple logos are often blocked out *because* they didn't license, they just used the product because it was aesthetically pleasing. Third there are whole movies devoted to basically being car company product placement. The Need for Speed movie was basically a dressed up Ford ad.
But lastly, Coke is easily the most placed product. It's everywhere
It is easier to write an incorrect program than understand a correct one.