If you walked by someone's house and their car was sitting in their driveway with all the doors wide open and a box of personal documents in the back seat, you'd probably knock on their door. If the car was still there after an hour, you'd probably knock again. I sent only two messages, and they were short and to the point.
(edit) What I mean here, is that to know that someone's door is unlocked, you have to check each house. To pick a lock, you need some rudimentary skill. Firesheep (and the underlying vulnerability) is wide open and requires 0 skill to operate.
It's one thing to politely knock on the door, it's another to keep banging on their kitchen window when they are obviously ignoring you. The users probably feel helpless and just want to be left alone.
For a non-tech person it's a pretty big jump from surfing Facebook at Starbucks to setting up a VPN.
(edit) What I mean here, is that to know that someone's door is unlocked, you have to check each house. To pick a lock, you need some rudimentary skill. Firesheep (and the underlying vulnerability) is wide open and requires 0 skill to operate.