The problem, I imagine, is that many games provide incentives for posting stuff on other people's walls. That's clearly spam, but also technically initiated by the user.
Send spam: "Click big happy green button to share!"
Don't spam: "Click tiny, scary 'X', which is followed by deliberately confusing confirmation, 'Are you sure you don't want to continue?".
They just keep asking you over and over; eventually you either mis-click or give in.
Then you get some real guilting. Your friends will miss you. Why don't you send some messages now to let them know you're leaving. Awful.