Telling computers and humans apart is a wrong goal. Every request comes from a computer that is commanded by some human. And why shouldn't users be allowed to use automated user agents when they don't do it for spamming or anything malicious?
CAPTCHA is essentially a proof-of-work variant where challenges are designed to be solved by humans rather than computers, and same as any PoW it works by means of consuming some limited resource (human time, processor time, energy).
CAPTCHA is essentially a proof-of-work variant where challenges are designed to be solved by humans rather than computers, and same as any PoW it works by means of consuming some limited resource (human time, processor time, energy).