Because the classes are multi-faceted rather than just one thing.
Fury Warriors aren't mindless berserkers. Yes, they follow the raging berserker theme, but that doesn't automatically mean they are dull-witted, rage-aholics, or adrenaline junkies, they're still Warriors.
Similarly, Arms Warriors aren't just mindless brutes wildly swinging a large weapon, they are also Knights, Men-at-Arms, Mercenaries, and brawlers. You name Garrosh/Grom, and while they might be brutish, even though both did some f'd up stuff, and inevitably fell in combat, it's hard to argue that they didn't possess keen tactical minds.
Classes in WoW (attempt to) encompass many fantasies under a few common themes. Inevitably there will be some conflict, but don't take each spell as indicative as the whole of the theme.
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Don't confuse the weapon with the class. Arms Warriors are not corrupted, nor do they pulse Shadow Damage, they both come from the weapon itself. While the idea of using these powers might not resonate with some people, keep in mind that weapons imbued with otherworldly energies is not exactly a new concept:
- Warriors wielding the Hand of Ragnaros weren't conjuring Fireballs, the hammer was.
- Warriors wielding Thunderfury didn't control the power of the wind to create tornadoes, the sword did.
- Warriors wielding Shadowmounre didn't control the shadow, powers of death or absorb souls, the Axe did.
- Warriors wielding Gurthalak didn't summon old-god tentacles, the sword created them.