"If a company like Reddit or Twitter derives most of its value from content that users write for free then it must provide APIs for anyone to download and manipulate that content."

I disagree that Reddit or Twitter derive value from content. They derive value from eyeballs and showing ads to those eyeballs. If people use APIs as a way to feed their eyeballs without looking at the ads, then Reddit or Twitter gain nothing in exchange for their massive expense of the hardware and staff required to run these services.

The other option is that people would pay to use Reddit or Twitter. But they would lose 90% of their users if they took that route, because what they do does not provide enough value that a typical user would pay for it. For most people, Reddit and Twitter are time sinks: mindless entertainment to consume to pass the time. In a prior generation they would be watching TV game shows or soap operas. They wouldn't pay $10 a month to use it, or even $10/year. They would just jump to the next best free platform and continue there.