Because I know MY cats do it.
>.> I left a glass of water on my dresser overnight and I wake up my cat's whole face was in it. I was like omg I was drinking after him this whole time lol XD
My cats do that whenever I leave a glass of milk unprotected.
^^ I always have to move food and drink away from mine, too. Even if they just sniff it, they usually end up touch it our getting hair on it.