"Bob's your uncle" has a pretty clear meaning

I actually still have no idea what it means. But the point is, you certainly can't deduce what it means from a literal reading. And I don't know anything about horses, so to my ears, "straight from the horse's mouth" has no inherent meaning, but my acceptance of it as an expression is not predicated on it being non-idiomatic.

As opposed to coming up with a completely different definition of the word "Extra"

Do you have any idea how many words in English have acquired completely different meanings and/or been 'bastardized'? Pretty much every word you ever use, since Modern English is completely disjoint from what it was hundreds of years ago. Sometimes the analogy used to generate an alternative meaning is clear (cool meaning emotionally cool, for example), and sometimes it is not (speaking of 'hot', when and why did that come to mean attractive?). Now, since hot has been used to mean 'attractive' for a long time, we accept that as an alternate meaning. But at some point it split off from meaning "high in temperature", and I can imagine a similar argument having been invoked over that change (note: maybe I'm wrong about the etymology of 'hot' here, but you get the point).

But otherwise claiming that entire groups aren't allowed to use random words because you've bastardized them (As is in the case of "Throwing shade") seems to not be logical in most senses.

Well now, that's a different argument. I certainly don't agree that entire groups aren't allowed to use any of these terms. Now, you're always going to come off a certain way when you use dialectical expressions like that, and that might include being seen as a try-hard, "wigger", whatever. But morally, I of course have no issue with people using terms coined by other races.