It's the power of suggestion. If you listen to it before seeing the 'lyrics' you'll hear hardly anything at all. When you hear it with the lyrics, you think you hear what you see.
And besides, think of this: Do you have any idea how difficult it would be to create a backward message in an actual sentence? He would have to have linguistics experts write his lyrics for him. And, I might add, the "Satanic" message makes just as little sense as the original.