Captchas are now useful to distinguish well-intentioned bots (they stop whenever they see captcha) from malicious ones, which solve them, but still behave a lot like bots.
CURL isn't a bot, it's a tool. It can be part of a bot (which may or may not respect robots.txt) but it can also just act as a user agent directly for a human operator in which case it SHOULD just do what asked. Chrome doesn't follow robots.txt either for the same reason.
