Oh, not this crap again.
If cross-genre influences were anathema the Arabian scale would be forbidden, as would elements from folk (so that's quite a lot of the black metal market axed, as well as all folk metal) jazz (several tech-death bands) neoclassical (Yngwie Malmsteen and countless power, black and tech-death groups), techno (all industrial metal) symphonic music (a vast swath of black, death and power metal) blues (a large chunk of traditional metal, many prog groups). Also, no DSO, Unexpect, Akphaezya, Karaboudjan, Myrath, Panzerballett, Thy Catafalque, or Sigh.
I'm so sick and tired of these slope-skulled quadrupeds grunting their inquietude about the 'purity of metal'. It's not a problem if somebody wants to listen to the same four albums rehashed over and over, but when they decry the defilement of their genre as if they're Angel Clare bemoaning the seduction of Tess it just gets tiresome. If they don't like evolution then the question has to be asked why they're still following metal news in the first place when they're clearly of the opinion that metal history stopped in 1994.