Sadly, I think I know what might have been the issue here.
You walk into a shop and ask for a waterproof rear light. Chap sells you a light which he says is waterproof. You ride through a storm, light gets soaked and dies, you go back demanding your rights etc cos you were told the light was waterproof.
Technically, the guy is probably right, the light won’t be 100% waterproof 100% of the time.
You may think I’m joking but I had a customer once who insisted that he’d been told that the lock he’d bought with his new bike was “unbreakable” so he kicked up a massive storm in the shop when his brand new bike got nicked after thieves cut through his “unbreakable”. In the end we sorted him out a new bike at trade but he still tried to take the matter much higher (as in, to the courts). The point is that if you really press a salesperson on how “waterproof” or “strong” something is, they won’t ever tell you it’s 100% waterproof. That doesn’t excuse the attitude obviously, but I can kind of see where he was coming from.