The main idea of this sign is that the Internet deprives us of real communication. Yes, we can chat, yes, we can make video calls, but most of the time while using the phone or laptop with the Internet — especially in public places—we seem to be in a cocoon. We don’t see or hear anything from the real world. I’m sure each of us had at least once a situation when he had to ask a person who told him something to repeat what he said.
Keeping in touch is cool. Keeping in touch with the real world is cooller.
And probably people in this cafe/restaurant understand this.