Natural monopolies are supposed to be heavily regulated though even if they aren't illegal. If you don't regulate them, they'll abuse the market by being monopolies. The fact that they are natural simply means it's an area where they are very likely to occur due to high cost of entering the market.

Like imagine not regulating a natural monopoly that provides electricity. They'll serve only areas they want and will not serve the rest. Good idea? Nope. Requiring them to serve areas equally is a proper method exactly because it's a natural monopoly.