"But there is just no law which says that if X has property A and Y has property A then X and Y must share any other property."

"X & Y both have properties A & B" is logically simpler than "X & Y have property A, X has B, and Y does not have B"

So if X and Y share property A, and X has B, this is evidence, by Ockham's razor, that Y has property B.