It's not moving the goalposts or arbitrary to pursue root cause. Suppose someone designs a machine they know will be damaged if it falls off a cliff. If they warn users of that machine not to throw it off a cliff, their instructions are not arbitrary, but based on their knowledge of the machine. They have a reason for their instructions. What would be arbitrary is for the designer to flip a coin as to whether to instruct users to throw the machine off a cliff when the consequences are known.
Saying nothing would be negligent.
Now, maybe the designer could make a machine that is not damaged when thrown off a cliff. In that case, instructions regarding throwing the machine off the cliff become somewhat irrelevant. Maybe the designer could inform users the machine will survive, but saying nothing is no longer negligent. It does become a rather arbitrary choice whether to say something because of the irrelevance of throwing the machine off the cliff.
You could say the decision to design the machine as cliff-proof or not is arbitrary, and I would agree. But it doesn't follow that because the design is arbitrary, the morality is necessarily arbitrary.
Even then, if one drives to root cause, I'm not sure if even design remains arbitrary. Not being an infinite being that is a question I can't answer. I do find interesting, however, discussions about mathematical first principles and whether they are self-evident or not. Mathematicians are always in search of such things. It is interesting that in geometry the Cartesian idea of an origin was challenged as arbitrary. The result was the origin used in affine geometry. Maybe you could argue the affine origin is still arbitrary, but the solution is so elegant that it begs to be accepted. It seems the arbitrary choice would be not to use it.