>one of the greatest heroes of recent Imperial history
>flaunts the use of a xenos weapon
Why hasn't some Inquisitor offed him because of this flagrant heresy?
When you're badass like this, you can get away with minor stuff.
Plus, I'm pretty sure he has a paper laying around where AdMech state how they sanctified the weapon so bow it is sort of kosher.
A big part of having hard rules is so that cool kids can break them.
If Yarrick didn't break the rule "Don't use alien weapons", then he couldn't also break the rule "You can't beat Ghazkull", now could he?
Tl;DR: Yarrick is the man.
I'm pretty sure the inquisition, for the most part, when they say "don't use xeno tech", are concerned with people fucking with eldar webway shit and obscure mind altering fuckery, not some totally mundane rusty piece of garbage that happened to have been made by an ork.
Considering the value of yarrik it's just not worth getting your panties in a bunch about.
This, it's only when it is tech the Imperium doesn't understand (which is most tech) there is a problem.
The Power Klaw is obviously a crude yet brutal weapon ripped from a Ork and grafted onto his arm as a fuck you to the Orks, and like already said, Yarrick is far too important.
Political capital is a thing. While Inquisitors have absolute power on paper (well, the Mechanicus would start making noises about the Treaty of Mars at this point, but ignoring that), in reality there are political considerations.
Yarrick is Hero of the Imperium and beloved by the Imperial Guard and the Commissariat. Offing him would be politically costly; souring relations between the Guard and the Inquisition, the Commissariat and the Inquisition, destroying moral on Armageddon. The puritan in question would lose face, lose influence and over what? A glorified power fist?
Much more important battles to fight, much more egregious tech-heresy to expunge. Not worth the effort.
People don't consider social/political enough. It's easy to do stuff in the shadows where no one notices, not so easy to kill heroes or popular figures. Look at Nimr al-Nimr. I mean, the Saudis did kill him eventually, but it took a long time, and that kind of backlash is not something any government or organization wants to deal with.