I don't love that this is opt-in by default, but I'm happy that they're at least offering an opt-out.
In this case, since the default is in unless you opt out, it's opt-out.
Like you, I would have preferred that the UI for the choice didn’t make opt-in the default. But at least, this is one of the rare times where a US company isn’t simply assuming or circumventing consent from existing users in countries without EU-style privacy laws who ignore the advance notification. So thank you Anthropic for that form of respect.
> We may use Materials ... unless you opt out of training through your account settings.
[1] https://github.com/OpenTermsArchive/GenAI-versions/commit/d8...