It's government that enforces the cable monopolies. They are called franchises, and it's the government saying only one company can run service to a given neighborhood. An EXCELLENT example of government doing harm.
I got the impression you were making the argument about the federal government specifically. Sometimes the federal/state government increases liberty by getting rid of a federal/state regulation. Sometimes abolishing a regulation leads to less liberty.
Neither I nor the Green Party believes government never does harm. I am certainly not claiming that federal, state, or local governments are free of corruption.
The core of the argument is that 1) government is not inherently bad and 2) we can substantially improve the quality of our government through 3) changes in electoral rules, campaign financing, and the revolving door. When a large voting bloc stops believing 1 and 2, we're basically doomed. I'd much rather argue over the best #3 and how to get them implemented.