I suppose that depends on the typical age of houses in your area? Just as supply lines inside houses might get contaminated, supply lines outside houses might get contaminated. Perhaps from sewage, perhaps from runoff, or whatever. Outside lines are more likely to be broken by sloppy excavation, for example. Simple isolation valves are sensible even in situations that don't require backflow preventers, such as on a private well.

I still don't really understand what you're saying.

Yes, I agree isolation valves are good. But as far as what code requires? It seems that if isolation valves aren't required on branches, then the only isolation valve that is required is the main house valve that can isolate it from the street water main.

So I can't figure out what else you think would have been required to pass inspection in the comment you originally responded to.