It's easy to judge parents from one experience like that. Some kids can be really wild, even after 3+ hours of play(ground) time, they show no sign of slowing down. And parents just want to have one meal in peace. In the end the tablet is a lesser evil.

Of course I'm not saying that it never happened that some parent abused the power of tablet silencing, but I bet that many parents using it are not happy about it either.