You took the words out of my mouth. For some reason I have never understood, book titles cannot be copyrighted. It's common to recycle old titles.
On the other hand, this is still an insult to a fine writer. He should have had the courtesy to at least try to clear it with Bradbury. Moreover, it's not as if the title really makes much sense for the movie; in that regard, it's a pure ripoff of Bradbury's original, with which Moore hypocritically pretends to have some sort of concern for free speech.
I'm guessing it is because of the usual brevity of titles. I imagine if Bradbury had spent the $1000 or so to trademark his title he would have had a much better case. Trademarks are often quite brief, as short as a few characters.