Oh, come on, is it really that big of a deal? The customer probably figured the carpet would catch any stray litter or mess before it hits the floor, which is actually kind of smart if you think about it. You just have to clean it more often, but hey, that's the price of having a clean house and a happy cat.
Used to be on your side until I had to rip up carpet that had been over a litter box for a year, the smell was unreal (like, eye-wateringly bad). That changed my mind real quick.