There are lots of citations:
"An appeals court on Wednesday sided with the telecom industry, and with North Carolina and Tennessee, in a major decision that upheld the ability of states to pass laws that restrict municipalities from offering broadband internet services."
And in Colorado:
And here's an argument for why they're doing it. It's all about never allowing taxes to be collected to be spent on something that benefits, well, anyone. Basically since government is inherently bad and can't do anything right, it will cost too much money and they won't work and only private companies have the "expertise" to run "an internet":
Putting up these laws as a reason for the limited deployment of municipal broadband is misleading. Most stages have no legal barriers, and for most of the states that do, the barriers aren’t really significant. They don’t explain why New York, LA, San Francisco, Baltimore, Philadelphia, Boston, Seattle, etc., don’t build their own municipal systems.