I think people are right that the motivation is profit, but I'm not convinced it's to manipulate you in any way. I don't think driving you towards SEO'd blog spam is really all that profitable to google.
My guess is that it's because it's an abusable feature, and that means hiring human moderators for it.
"Sorry! Looks like something went wrong. In order to use our site you'll have to enable idle tracking. This helps us improve our software for customers like you!"