As Bill Duellman, reknowned herpetologist, writes in the introduction to several of his books. The typical life history of frogs that lay eggs in ponds and then leave the eggs to fate is not really so typical. The majority of frog species live in the tropics and in wet environments, there are many benefits to laying eggs on land and either transporting the tadpoles or allowing them to drop in a suitable body of water which is small and predator free as the previous post explained. Dart frogs actually carry their tadpoles on their backs and haul them to suitable places to grow. Often these suitable places are those tiny cups of water you are wondering about. In the grand world of frogs. Dart frogs are not so strange afterall. It's those bullfrog that are weirdos.