I think it's truly pixie dust and just depends on the housekeeper you get. I don't think tipping really matters...we always tip, and never got towel animals until this past trip to WL, where we got one every day!!!!
My in-laws didn't tip at all, but they got a few too...I think we just got lucky this past trip and got a great mousekeeper. We left them in the room, thinking they were hotel property, but when she saw us walk by on check out day, she told us to make sure that we took them with us, which we were happy to do. DS got a Birthday cake towel, we got an angel, a candy cane and a pair of mouse ears!!!
Come to think of it, this is the first trip that we've actually seen/met out mousekeeper. I wonder if our just being friendly to her had anything to do with it.