Can't speak for the other commenter, but I'm "available" when I sit at my computer playing Civilization for 6 hours, despite the fact that I won't touch my IM application that entire time. If you're trying to set "Away" based on how often I interact with the application, you're getting it wrong for 5 hours and 40 minutes of that period.

I don't see what's not to understand or what logging out has to do with it. I have a browser tab for an IM application, that's also available as a native app on my phone. That tab is always open on my desktop. If I have other tabs open and am doing stuff, I still want my IM status to be 'available', and I want all notifications about new chats to go to that tab and to no other devices, while I am on my computer doing stuff.

If I go out for a walk or I go watch some TV or otherwise stop using my computer, depending on the chat app I want to either automatically be 'unavailable' (for work chat app), or I want to stay 'available' but have notifications go to my phone instead of the chat tab (for friends' chat apps).

His "special case" is that he goes more than 20 minutes between checking his chat client, but he is still available, so why should he be marked as 'away'?