It absolutely is how criminal laws work under the Constitutional prohibition of ex post facto laws; while we can write forward looking criminal laws however we want (within other Constitutional limits), we can't apply those new laws to past conduct.

You can apply them to discussions about hypothetical solutions to current problems, though.

No, that's not how laws work. The law comes first. Then its application to behavior. If they didn't break any existing laws then there's nothing to do but propose a new law.

So, what, it should be illegal to leak your data on public Facebook scraping?

None of the info they had was private info.

If you don't want your info to be leaked then don't make it public.

Not retroactively, that would be a disaster.

And specifically mentioned in the constitution (twice!) as something the government can't do.