While I think the OP may be pulling the trigger a little early on making his snake pass waste, they do not all eventually poop on their own. Impaction does occur in snakes, and not only from substrate ingestion, which makes it difficult/impossible for a snake to pass waste and can, in bad cases, lead to death. I've also dealt with a snake a few years ago that for a while was developing super hard urates that could not be passed without assistance, which could have ultimately led to the same result as impaction. So no, they don't all eventually poo on their own.