Why in the world would they do that, when they could simply use another tune at no cost?
What other tune? Was there another space blockbuster from the previous couple of years that MGM had the rights to? (Especially one that would go with a five tone code?) It was a very up to the minute joke.
So they swapped.
I mean... Not exactly swapped. Those movies are six years apart. Spielberg granted them the rights because he's a Bond fan and he could afford to be gracious. It just happened to pay off for him. And I'm sure Williams got a few bucks for it.