LTS, it is also because the ammonia smells to them like a rival cat. Gotta one-up that bad invisible kitty and remark the territory.

They like to pee on plastic grocery store bags too. If you have one peeing in the wrong place, line their kitty pan with these bags, it will attract them. After putting this near where they have been misbehaving, put down their cat food where they are inappropriately peeing or pooping-- they will pick a new spot, eventually learning that the whole house is a potential food dish, not a cat pan.

~S~