..Why would the DOJ do something about the seizure? The DOJ is leading the investigation.
Also, just because something is unprecedented, doesn't mean it's somehow wrong or shouldn't be done.
The FBI went in to look for any remaining documents that were not for Trump to keep, they found them. Trump is quilty of that, there's no question about it. Thanks to Trump's big mouth, we know he was aware of them. That there turned out to be highly/classified information is just adding to the pile.
Btw, Trump's lawyers are asking for security clearance to be able to have a look at the documents. Strange, considering Trump says they're declassified.