(AP) — When vandals sliced a fiber-optic cable in the Arizona desert last month, they did more than time-warp thousands of people back to an era before computers, credit cards or even phones. Because Internet service is largely unregulated by the federal government and the states, decisions about network reliability are left to service providers. Despite its own warnings about such vulnerabilities two decades ago, the federal government has taken no steps to require Internet companies to have backup systems, even as it has provided billions of dollars in subsidies to expand broadband Internet into unserved areas. In northern Arizona last month, tens of thousands of residents were without Internet service — some for up to 15 hours — after vandals cut through an underground bundle of fiber-optic cables owned by CenturyLink. Companies have been deploying more than 10 million miles of fiber annually in the U.S., increasing the risk of damage from backhoes, trench-diggers and shovels, according to an analysis by a network reliability committee of the Alliance for Telecommunications Industry Solutions. Since 2009, the U.S.