About "calculation" CAPTCHAs

During one of my PHP security sessions, I was asked about “calculation captchas”. These look like this: and the Turing test (if you want to call it that) is solving the task in the picture.
But wait, isn’t it the main function of a computer to perform calculations? So how can this be "Completely Automatted Public Turing test to tell Computers and Humans Apart?

Just for shits and giggles, I downloaded 100 of these little images from the site that I saw them on, and fired up gocr. I piped the results into bc and voilá:

18 - 8 = 10

13 - 5 = 8

13+4 = 17

13 - 5 = 8

16 - 2 = 14

19 - 3 = 16

11 - 3 = 8

13 - 5 = 8

13+2 = 15

14+4 = 18

15 - 4 = 11

10 - 4 = 6

11+1 = 12

10 - 2 = 8

82% of CAPTCHAs solved on the first try, with plain (untrained) gocr and a one-line shell script.

for i in *; 

do 

    calculation=`gocr $i | sed -e ‘s/=//’ -e ‘s/\?//’`;

    solution=`echo $calculation|bc`;

    echo $calculation “=” $solution;

done