It would have been interesting if any of those users who got their video file deleted would have been directly attached to the shooter (like an accomplice, let's say), afaik that would have meant that Google Driver and Dropbox had tampered evidence.
If a judge said that owning that video was probable cause or whatever, they could produce every user who used to have that file.
On an aside, such a setup is also a layer of defense-in-depth protection against a certain, uncommon class of bugs.
I do work for Google, but don't, in any way, speak for the company. All of the above information can be found on the internet, eg. https://www.quora.com/Does-Google-really-delete-the-account-... about Google accounts themselves.