I think a lot of nsbm bands are simply reactionaries pitching a gimmick. But some just preempt the branding, if you deal with racial, environmental, cultural and nationalist themes you'll be branded a nazi so you might as well just go with it. Unfortunately most nsbm bands suck and the only reason they get any attention is because of their nazi stance (Graveland, Thorrs Hammer, etc.) as opposed to a lot of good bands in other genres (neofolk, industrial, etc.) who are actually good and are held back by their perceived nazi sympathies (Blood Axis, Allerseelen, etc.). Most nsbm bands donít actually champion the political platform of the actual National Socialist movement and are for the most part in direct contradiction on most things. It is an unfortunate label that should be abandoned by any band that does want to achieve anything positive. You can promote PanEuopean Nationalism and Aryan religion without falling into the trap of Nazism, which was a perversion of these themes anyway. But at the same time I can see how and some bands would play with this taboo to avoid being commodified and exploited. And then again, metal is all about taboos. But you can do this like Slayer has by exploring these themes without endorsing them.
But if you really want to check some of these bands out, the only one that I have ever come across that was any good was an Australian act called Spear Of Longinus. You can find them on Youtube and some file sharing sites.