“It’s because the law gave them too much leeway” doesn’t make sense. If you have leeway, you choose the easiest and cheapest method, not the one that involves building a whole new communication system.

“They thought it was the only way to satisfy the law” also doesn’t make sense. There’s no reasonable way to get that from the law, they didn’t confirm it was necessary, and they didn’t publicly fuss about being forced into a large expenditure.

Today is, by the way, the one year anniversary of the repeal of net neutrality. ISPs, including CenturyLink, were viciously fighting to end rules preventing the sale of selective internet access, and won. In order to sell selective internet access, you need exactly the system that just showed up. This system would take a while to build—maybe a year. And once it’s built, you would want to test it. The test would put your capitalization on the lack of net neutrality in the spotlight—unless you just serve a legal notice, and pretend you thought you had to. For that to work, you need to make sure people know the “legally required” part; for a legitimately legally-required notice, companies typically don’t say as much. CenturyLink literally highlights it.