That's incorrect, and at this point, it seems like you either don't know Medium or are engaging in gaslighting here. The Medium paywall is not always a choice, as Medium has placed many stories behind it without the creators' consent and doesn't pay everyone either. Furthermore, you guys display those paywall ads on all articles now. I'm not referring to the OP's case but speaking generally. Although it does seem that Clive, being a blogger, is apparently under a secret contract with Medium to write premium content for the website.

The paywall is definitely always a choice and the choice is only available to people who get paid.

Then how are my stories marked as member-only when I had never enabled the paywall. It doesn’t seem like a choice for writers.

Can you give me a link to an example?