I find this really interesting. I wonder what other restaurants would make out of this. Is it really that important that the diners do not use their phones while dining ? Does that make them orde rmore food ? How will this 5% discount create more ROI for the restaurant?
Admit it you are already wondering what restaurant this is and what they serve. If that it is the case, they have done their job well.
But taken at face value, it is about the principle. It will push people who like to talk on the phone and disregard everyone around them to either not show up (or it will prompt them to assert themselves and they'll show up and pay 5% but in return will make sure to talk constantly on their phone, so, could backfire).
The apparent intended benefit is to attract many who have been around annoying customers and who would also visit the restaurant out of principle.
It would certainly be interesting. Also, wonder why they can't just ask someone to leave...?