"So someone who doesn't understand calculus or advanced math is an idiot then?"
Well, calculus is 300 years old, and is not exactly advanced maths -- it's fundamental and basic -- and it's the numerical equivalent of knowing who Tennyson is. In the UK at least (and I assume the US), knowing anything other than basic arithmetic (i.e., adding and multiplying single-digit numbers) is considered advanced, and this is generally innumerate people's way of trying to cover up the fact that they have a severe and gaping hole in their education.
To see what I mean, replace "can't add up" with "can't read". People who can't read in the UK are either three or are considered mentally retarded, but people who cannot do arithmetic are just "not good with numbers", despite arguably arithmetic being a much more natural thing than language.
Maybe this calculus problem, which actually is a fairly easy calculus problem (differentiation is a science, integration is an art) is an unreasonable captcha, but this post smacks of society's general belief that mathsy stuff is hard and dull, etc..
(On a totally unrelated point, I remember getting one of these captchas ages ago for some forum, but I cannot remember which one...)