We have a lot of sport crags in the mountains, and most are bolted. But sometimes a cliff has plenty of cracks, and the developer has specified the routes as trad. Over the years, folks have respected this. They still install bolted anchors, however, for safety.
But a more problematic issue is the amount of traffic. The more folks climb a route, the sooner you'll see an accident, and safety becomes a numbers game. So a really popular crag should be bolted, while on a less-climbed route, it's your choice.