The problem is that the CAPTCHA approach is flawed. Any similar type of challenge-response system can be abused for illegal activity.
I met a guy who was a pilot in Vietnam. They had (and still have) a system where everyone carries a card with a grid of numbers and letters on it, and you can authenticate someone over the radio by picking a couple spots on the grid and they respond with, for example, the character adjacent to them. Well, he forgot his card one day and was queried by a controlling agency using the authentication card. He told them to stand by, switched frequencies, and issued the same challenge to another agency. They responded, and he switched back and passed it along to successfully authenticate himself.