SATANISM

[MR. Macabre 666]

Because there are dozen's of other bands that are better, heavier, darker, and more authentic than any band who's shirts you can buy at Hot Topic. I've been listening to black/death metal for over 20 years, and "traditional" metal bands for over 20 years before that.
Whitechapel is a metal band?
If you're interested in hearing some real music, try this short list and let me know what you think,
Emperor
Immortal
Watain
Behemoth
Inquisition
Dark Funeral
Batushka
Hypocrisy
Morbid Angel
Dimmu Borgir
This would be a good start.

Genuine question here, I'm not trying to accuse anyone:

To what extent to bands like this use Nazi or White Supremacist imagery? 

I know that some people who don't believe in a real Satan use Satanic symbols (partly) for their shock value. It's fun to get a rise out of little old ladies. 

Is Nazi stuff used for the same purpose, even if people don't particularly believe it?

What you're probably referring to is a genre of music that's labeled NSBM(National Socialist Black Metal). None of the bands that I've listed have anything to do with those subjects, and I don't/won't listen to any of them. There are a few bands that take it seriously and are on very obscure record labels. Luckily, they're few and far between.
I'd have to say that the majority of the music I listen to is by musicians that take their subject matter very seriously. Watain, Behemoth, and Inquisition are great examples of this, everyone in those bands is an avowed Satanist who have been doing it for a very long time.
Behemoth just acknowledged their 30th anniversary as a band.