I've had guests who thought that the fruit bowls, or even the water carafes/glasses were 'for decoration' so didn't touch them. I guess a note saying 'help yourself' might have done the trick, but I had thought it was obvious!
I do host private rooms in my own home, so I think it's absolutely right that guests do not just help themselves to whatever they see. In fact, they are specifically told not to do this and shown what is for their use in the kitchen (another thread running on the CC at the moment RE guests who ignore this).
I'm talking about items left for them in their rooms, so I thought that should be obvious, but that's not always the case. Weirdly, guests seem to be more likely to help themselves to other people's things in the communal areas than to the things specifically left for them in their rooms.