I empty my small kiddie pools every day or two, depending on my mood. Every few weeks, I take an aquarium sponge and scrub out the algae (I don't obsess over it though--they are never pristine). They do get them filthy, and they do love them clean. In their natural habitat, they'd ideally live in large bodies of water occupied by an entire chain of life that would process their mess quickly. It is natural for them to want to bathe in clean water, but to also settle for filthy water, and to have no concept of how to keep their clean water clean.
Emptying the pools has a couple side benefits. One, I use the water to water the gardens and fruit trees (sometimes), and it is GREAT fertilizer. Two, moving the kiddie pools regularly prevents them from burning brown spots in the grass. Of course, it does not prevent the ducks from digging holes in the mud around the pool in each new location. lol.