Forgive my post if it seems lacking in knowledge.
What would an autopsy even be on just a few bones?
*ME: walks in, sees a few bones, "yep, He's dead"
Is it just a legal requirement or something else
Oh, goodness, yes!
Marrow chambers in bones reveal whether the person inhaled water before they died.
Microscopic analysis tells which critters gnawed away at the bones (water, by itself, isn't such a good decomposer - it's just the soup, we need to know the diner).
The diners reproduce, eat dinner and give lots of evidence of their life in this ecosystem (BL's bones).
An autopsy is supposed to use the best means known to the community in which the doctors (particularly pathologists) live. There's tons of stuff to be known - even from bones that are thousands of years old.
Australopithecus bones (our ancestor) from 3 million years ago still show who the predator or scavenger was, who ate/came upon them. Or whether it was human-inflicted.
Every bone should be examined for cut marks (various kinds) and weapon injuries (various kinds).
In most places, when there's a death of a person who isn't elderly and with a known medical condition, an autopsy must be performed - so yes, it's a legal requirement (because in general, we don't just ignore people's deaths and don't try to figure out what happened).