If they allow users to delete or edit their own content, then what's the issue? That just sounds like a technical problem StackExchange didn't think of.
On the other hand, why is this behaviour deemed unacceptable, and StackExchange double dipping on user generated content is not?
At least I'm engaging on StackExchange with the understanding that conversations (questions, answers, comments) there go on the public record for the benefit of future curious humans. I see that as a way of giving back to a community that's provided me with invaluable knowledge.
If I'd known somebody might delete their half of a fruitful conversation as a political statement months or years down the road, I might never have spent the time and effort of participating.
Thinking of it in terms of co-authorship, one of several co-authors also doesn't get to retroactively destroy the combined work by ripping out their individual contributions. If you collaborate, make sure you know and are aligned with the terms of the resulting work.
I don't agree with how StackOverflow's current management has been running the site, but for anyone who has used the site, the outcome of this behavior should have been obvious. I'm sure Wikipedia would react the same if you deface all the pages you contributed to prevent OpenAI from consuming words you wrote.