The simplicity of it is logic defying. It used to be that you had to find crosswalks or move puzzle pieces or type blurred letters and numbers, but NOW all the sudden I can just click a box and HEY!, I’m human?
That’s hardly the Turing Test I’d expected.
Others mention the mouse motion, and monitoring your other traffic to similar sites. When it shows the checkbox, it has already determined you are probably human. If you had suspicious activity, they will give you more advanced tests instead of just a checkbox.