There are lots of different kinds of curfews and reasons for them,
There is NO North American or US curfew.
A curfew can be a local law, usually in the US this is a noise ordinance rather than a ban on personal behavior. If a venue is close to a residential area for instance a curfew prevents concert level noise at a time when peole would be sleeping.
Another type of curfew is actually a union rule that governs how much a band has to pay for the services of union laborers and will have a much higher rate, sometimes double or triple time, for all the union workers, both on the production staff and local if the show goes on past a certain time.
Some venues have a curfew especially when the venue has another event the following night. In the mind of the venue the sooner the artist has all thier shit packed up and gone the sooner they can turn over the venue to the next artist and if they aren't ready for the new artist on time there are financial penalties for the venue and in turn for the previous night's artist.
In some areas a cufew is a local law against large events going on past a certain time. Usually this is done to prevent too much overtime for the large cadre of law enforcement officers who are required for crowd control, traffic direction etc. Every extra 1/2 hour of concert is an extra 100 hours of overtime and lost sleep for the local PD.
So anyway, lots of reasons that make sense for a curfew and in any case a concert can start earlier if the artist needs to play for a longer time.
Dave