I think a lot of these devices operate directly via a network, some I believe even operate via wireless. Regardless of HOW they operate, there will always be an attack vector.
There is nothing to mitigate human error. We all make mistakes, let alone some of us being plain dumb.
Pennu.ru do not know who or how the display was accessed and aren’t ruling out a competing advertising company being involved in this eye-opening event.
That is a ridiculous statement. They aren't ruling out a competing advertising company, but that doesn't mean they have any proof at all.
Photo of the billboard screen (NSFW): http://damian.livejournal.com/596391.html