I believe, sir, you may have answered your own question here. There's no difference in the two groups of musicians except that we agree with the lyrics one group uses and disagree with the other. I frankly don't know why Evanescence or Creed use so much Christian imagery in their lyrics, but both groups readily admit that they are not believers. I don't know why Ozzy uses so much occult imagery in his lyrics, but he's been equally honest in interview after interview saying he's not a Satanist.
One must be consistent.
On the other hand we have bands like Black Sabbath that sing in support of the occult and even recognize occult figures like Aleister Crowley in their music. Crowley is not a casual figure in the history of the occult in the twentieth century. From all accounts, Crowley took himself and his work in the occult quite seriously, and though he may have made a living from his chosen course of study, he could not be said to have been in it for the money. And most assuredly not in the Victorian United States of the late nineteenth and early twentieth centuries.
So why do bands like Black Sabbath have such a loyal following? Is it because they support the occult in their lyrics, or is it that their music is superior? Or both? And why would an individual, if they did not agree with the tenets of a particular belief system, promote those same tenets in their work? Especially if they opposed another belief system to which the individual had already professed fealty?
Let me ask you another question that may appear to be off subject. Pretend for a moment that you are a salesman. The principle often referred to as the 'Golden Rule', that you should treat others as you would have them treat you, is ingrained in your character. You enter into this new profession with hope and enthusiasm, optimistic for success. Sales don't happen as quickly as anticipated and so you seek out the most successful sales representative in the firm for advice. Before dismissing you from his office he says he will share his secret with you. Pushing his chair over to yours he looks you straight in the eye and says, "When I meet a customer for the first time, I use the initial meeting to try and size them up. If I get the sense that they're the religious, Bible thumper type, buddy I'm right there with 'em praising God. But if they love to carouse and raise Hell, then boy I'm cursing and laughing when they take the name of God in vain."
What is your opinion of this man's character? Is he someone to trust? Would you accept an offer of tutelage from such a one? Is what we take in through our eyegate and eargate any less important to our health and well being than the food we eat?
Actually, both Amy Lee (Evanescence singer) and Ben Moody (ex-guitarist) are both self identified Christians. Ben Moody is very much a believer.