I used to have an internet connection from a small ISP that used carrier grade nat. Same issue. I think most of these captcha systems basically just look at IP or other reputation, and then make end-users do mechanical turk style work for free.
Someday, I'm hoping some sociologists look for evidence of socioeconomic discrimination in captcha implementations.
In my experience, performing the exact same actions with your mouse in Mountain View leads to a completely different outcome than it does in lower income areas (red-voting white, ethnic minorities, etc) surrounding the Bay Area.
