One of my friend's cats (Buffy) does this. It is the only non-litterbox place she goes. I always have pee-buddies (there are two boxes in the bathroom off my bedroom) and whenever I go, one of the cats is sure to join me in their box. If there is no other behavior change and no other non-litter peeing going on, I would not worry about it.... As you say it is easy enough to clean and I think the cats are just going as they perceive their humans are.