Auto log-off

What is the deal with sites the automatically log you off after a certain period of idle time?

It is ostensibly a security feature, but I find it annoying as hell. I log onto secure sites using a computer that I control. How does auto logging me off make me more secure in that case? If someone is too stupid to log off their online banking account when accessing from a library computer then maybe they deserve to have their security compromised.

Does anyone have any insight as to the motivation behind this dubious feature?