My cats do something weird too. If a tiny spec of dirt or a bug or anything gets in their water they won't drink it, but they will drink out of the toilet.
I observe the same thing not with a cat (because I don't have one), but with our dogs. Clean water is always available for them to drink, but they sneak and drink water from a pail in the toilet or in the garden.
They know it's taboo and that they're getting away with something. They're like little kids (or men), if you're trying to stop them from doing something they're going to want it more.
My cat has two bowls of water, one in the kitchen and one upstairs. Still he will sit by the bathroom and sink and wait for somebody to turn the water on. Then he sticks his face under the faucet and tries to drink.
