That's exactly why I don't see the reason for getting so upset about it. There are probably a hundred restaurants within 10 miles of this place that don't ban kids under 6. Odds are this will increase their business, not decrease it, and it's not like it's a chain. No, it isn't kids' fault that they don't understand things, can't control their emotions or volume, etc, but there are tons of places to go with your kids, both behaved or not.
What this sounds like to me is people going "But MY kid is different, MY kid is special and well-behaved, so why can't MY kid go in the restaurant?" Your kid isn't special; get over it.
And then, 6 years old isn't some magical behavior problem cut-off. Some kids have screaming tantrums as teenagers. Some kids can't behave regardless.
The first time I can remember going to the movies was age 3. My sister and I were chatting like idiots, but then my dad (mom wasn't with us that night) yanked us both close and said this was a special treat, and if we didn't stop talking, we were going home right then. My dad never fucked around with false threats, so I listened and even my 2-year-old sister shut the hell up and watched the movie. And that stuck from then on.
Point is, if you teach your kids that going out is a privilege and that they'll be left out of it unless they behave, great. This restaurant wouldn't need this policy. But there are still all the other restaurants in the country to go to. It doesn't have to have a ball pit--it just doesn't need to be this place.