This is a more general question:
Why are people (metal fans and non-metallers alike) more prone to lose their minds when a band is openly hateful against Jews or any other race/religion, when hating Christians and liberals normal for other bands in the same field?
Social programming. Logical inconsistencies are probably considered a feature.
There's an implicit assumption that Metal participation entails some type of 'otherness', yet subculture participants are often more conformist than average, due to their strong desire to appropriate an external identity. The younger they are the worse it is.
Also, it's fairly standard practice to retroactively impose ideology over arbitrary youth subcultures. The true underlying patterns that bind a subculture together are often entirely implicit with a complete lack of explicit rules (probably due to them forming from the bottom up). Participants develop an intuitive feel for what works and doesn't when socializing in the group. Anyone sufficiently adept at manipulating the implicit rules, increases his in-group status, and is free to impose semi arbitrary explicit rules. Disagree? leave our club poser/hipster/personwhoisn'tauthenticlikeme.
And by the way, "political" music from the right tends to be fucking terrible. They rightfully treat it as a generic propaganda platform and produce things via template without any pretense of art. While the left seems to produce narcissistic love ballads dedicated to whatever darling idea they are currently fetishizing.