Why do people complain about moral relativism as if their own morality is anything other than arbitrary?

Actually, I don't get mine from a scripture. I'm pretty sure I got it from my parents. And they got it from theirs, and they got it from theirs...etc, etc. In my opinion, the Bible just confirms that we're on the right track.

And anyway, really and truly, morals are subjective.

We get it from God, not from scripture. Scripture merely confirms that such morales are from God, as they reflect what is already in the hearts of individuals, put there by God.

Of course, you might be better served by studying some history, as well as some Greek philosophy on this point - looking into the role and necessity of a moral lawgiver to explain the existence of moral laws to begin with. Arbitration doesn't explain the existence of such laws, only why they are so different for those who reject the standards God has set out.

When one gets something from 'scripture', it still means no more than opinion. And everyone has one.