Why government regulation? To prevent price gouging, favoritism to particular customers, killing off rivals. Prior to the Carterphone Decision you could not attach any third party equipment to the phone lines, including phones and modems. There was a huge battle for MCI to attach their long distance phone lines to the AT&T system. Common carrier status was invented to allow people to use their phones as they wish. When I was a child coast to coast phone calss were several dollars a minute. Now unlimited long distance is the price of a single call of the past.
(I realize that I answered the question "Why is regulation necessary?", but it goes along with the sanction.)
The concept of common carrier status had been around for a very long time when it was first used to apply to telecommunication services. These days, phone companies and many kinds of physical transport services are common carriers, while ISPs have only the benefits of common carrier status, and none of the limitations.
In order to try and regulate it, the federal sanction put Bell under purview of the FCC, so the FCC could handle Bell directly instead of having to go through the Justice Department every time.
While it ultimately failed, there was a method to the madness.
If it was natural, why was a government sanction necessary?