People really have a highly selective ability to be annoyed by things that they are judgmental about/think that they are better than -- or even worse that they think make people better than them -- especially if it triggers their jealousy/resentment. If someone is getting texts or calls, that's incredibly annoying to someone who gets no texts or calls for obvious reasons.
Some years back we took our young children out with their grandparents to a family restaurant. It was mid evening on a Saturday night and the place was full and very loud, with escalating conversations, people having fun, etc.
My infant daughter dropped her fork. I happened to catch a woman nearby -- likely barren and fervently vengeful -- giving the most hateful, glaring look I've ever seen, and I would imagine she went forth to declare how her night was ruined by our much quieter than average table. I recall this experience whenever I hear some railing about children on airplanes, etc -- I imagine that much of it is selective, desperate clutching for offense, and it's much the same regarding people and their anti-smartphone spiels.
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...?