This thread (linked below) explained in great detail the problem of unknown dogs meeting each other and reacting to the other dog aggressively or even savagely.
Posts #6 and #8 give good insights and Post #6 seems like an excellent way of introducing stranger dogs to each other. But "meeting in neutral territory first" is not going to happen as people bring in stranger dogs into a crowded or cramped and busy restaurant patio.
So how then, do people expect strangers' dogs to meet each other in a restaurant patio, under foot of patrons and servers, without aggression while many owners have done little or no training/socialization of the dog?
After seeing a big, leashed dog on the sidewalk diving under the railings of the patio at Johnny Rockets to viciously attack two docile small dogs just resting under the table, I believe the potential for dogfights in a restaurant patio, under patrons/servers' feet, is a huge liability and a terrifying sight to see.
For Dog owners only: How to introduce your dog to another dog, and how not to.