some areas of business are just natural monopolies
"Natural monopolies" — a pro-government excuse like "market failure". If Tokyo has competing subway lines, why can't New York City have any?
Well, why don't you go build one? Just spend a few billion of your own money - I'm sure you'll make a profit in no time, despite not being able to charge your full costs due to the presence of an established company that has paid for these sunk costs.
Regulation is required anytime you have a monopoly, no matter how it got there.
The primary focus of the "regulation" is to try to ensure the presence of healthy competition — which is by far the best regulator there can be. No government-created monopolies (like AT&T's) and no duopolies either, please (as there were with cell-phone service in the 90ies).
Thus, it does matter, "how it got there" — if it was government-orchestrated in the first place (as AT&T was), for example, it may need to be forcibly split-up. If it grew up on its own (like Microsoft), it just needs to be watched so that it does not use its monopoly position to against competition.
I agree that one of the best ways to use regulation is to create competition, thus minimizing the scope of regulation.
However, I don't think the origin of the monopoly matters at all. Obviously if a monopoly resulted from poor regulation that should be fixed, simply because it is poor regulation. However, all monopolies will tend to use their position to block competition and extract maximum economic rent. That is just the nature of business. It is only the fear of regulation that might cause companies to avoid it at all. If they didn't do these things in the US the directors could probably be sued for it.