If Tesla can’t make a chassis that keeps rainwater away from the battery, then they shouldn’t charge their customers for it when the battery inevitably has water ingress
This sort of issue happens on every type of car. It's not routine that a Ford F150 has a cracked block that leads the engine to hydrolock, but it does happen, and Ford doesn't cover the damage 8 years later.
E.g. if Tesla batteries only last X years and you plan on keeping your Tesla for Y years, then knowing that leads to better decision making.