Isn't their monopoly "brought to you in part by violent corrupt government!"?

If government didn't exist, diamond mining corporations would just hold town hall meetings with indigenous people. They would send their best and brightest sales force out to conduct presentations explaining how much money the people who agreed to give up their mineral rights could make. These corporations would use third-party arbitrators to draft easy-to-understand contracts in the native language of those indigenous populations. They would buy airtime and have their best PR people explain to the public consumers how great this is going to be for everyone, namely the indigenous mineral owners. Once the agreements are signed, the accounting and legal departments of this corporation take all of the contracts and related documents and publish them online for the world to peruse.

These corporations would do this despite the fact that it's cheaper to pay these people fractions of a penny on every dollar, convince them to work in the mines, and hire men with guns to keep the peace.

Why?

Well, because corporations you've never heard of that supply a significant portion of the world's diamonds like Rio Tinto and Debswana wouldn't do unscrupulous things to indigenous populations without the existence of government. Literally, they only behave this way now because government exists, and they're not simply using the cheapest tools at their disposal to achieve the highest profit.

These people just can't fathom that, sometimes, using violence and coercion can be more profitable. To use their logic, though: since there's a current demand for coercive and oppressive agents (in many modern cases supported by tyrannical government regimes), one would be right to assume that both a demand and supply will exist for coercive and oppressive agents in Libertopia or Ancapistan.