I know that if I were bringing my well behaved child out to a restaurant and was told that wasn't allowed then I would be unlikely to take my business there again, with or without children

For sure. I suppose they weighed the options and thought they would be able to make it anyway.

The thing where you have never witnessed a child being so badly behaved that they were asked to leave has probably never happened because the staff were afraid to do this sort of thing. I worked in restaurants in the past where kids would cause absolute mayhem in the dining area and left a huge mess behind afterwards. I think if I were a parent I would probably do like you do and do something about it..

You are right that it is not fair that all people with children be banned for the actions of the few but what other choice do they really have? I suppose they could put up a sign that says that this sort of thing will not be tolerated and people will be asked to leave if it does but I can also see that situation getting wildly out of hand if the parents are also unruly.