Back when you could get a steam key for any price, people used to spam 1 cent purchases in order to get mass amounts of steam keys. I think this is left over from when that was possible.

I don't recall ever having any issues myself when donating for a Humble Bundle, but I frequently enter the wrong characters on other sites' CAPTCHAs, so I totally understand your frustration. Actually, I think it's because I'm always (just barely) beating the average: "Orders below the average must authenticate with reCAPTCHA to help cut down on costly abuse." Is this the CAPTCHA you're referring to?

No, I paid above the average. I was given the CAPTCHA when trying to log in to my existing account.

The captcha is a joke. You're paying a dollar or less for the game, so because a third of that goes to charity, and you're not willing to give more, then they just want to make sure that there is a human with a soul at the other end. Because all data says otherwise.

No, I got the CAPTCHA when I logged in. I paid above average and sent it all to the EFF.

That's weird. I just bought it with paypal and I didn't see any reCAPTCHA.