Another in-character reason is that T'Pau thought it was blazingly obvious that when you fight the groom at a wedding to take possession of his bride, you'd better kill him. It would have struck her as silly, as having to spell c-a-t, to mention that the fight is to the death. "This is for all the marbles. Aren't weddings done like this on every planet?"
But T'Pau isn't some uneducated hick from a backwards planet. Not only is she from a well-educated and well-traveled culture, but she's known enough in Federation circles that she was offered a seat on the Council. She has to know that different cultures do things differently, plus we see her explain several OTHER things to Kirk and McCoy.