(This is similar to how LucidChart works)
https://prezi.com/pricing/basic/
The basic free tier for Prezi is "you can use our platform but your presentations are public". Once you're not on a paid plan, there are only two choices: you can delete your presentations or make it public.
When you are on a paid plan it IS possible to download your presentations to your computer.
There's nothing that entitles you to "my saved presentations are mine forever in Prezi and they stay private even though I don't pay.". Nothing.
I’m not saying that anyone is entitled to it, I’m just saying that I think the model of ‘content created when I was a premium subscriber can retain some limited premium features’ is a much more graceful and customer-friendly downgrade path.
Forcing all your documents into either public, or a downloadable format you will never be able to edit again even if you resubscribe, doesn’t seem very consumer friendly.
a) delete all your private content; b) make all your content public.
But the tweet is tendentious. Nothing prevents the user from deleting their content - THAT would be the problematic part.
So it's a small nuisance for the user, but they ARE telling the user. They don't make your content silently public and then say "you agreed to that in paragraph 7.9.12 when you registered."