Those are two horrible examples.
Titanic is called Titanic because it is about the experience one woman had surviving the sinking of the ship, and the romance she had while a passenger on board.
Pirates of the Caribbean is called Pirates of the Caribbean because its named after a Disneyland ride, and also because Jack Sparrow and his friends ARE Pirates in the Caribbean. Therefore its about Pirates of the Carribean.
Neither movie is about Tron and he's just a supporting character in both
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"One of these days, I'm gonna find a loophole my friend." - Man in Black