I would think that all these "copyright" scans Google performs are not performed against a list of known-infringing files that Google researchers compiled themselves by monitoring pirate websites. Instead the known-infringing list would be compiled from previous takedown requests. And one of these takedown requests either explicitly or implicitly (e.g. by listing a "folder") contained a .DS_Store file that looked like many other .DS_Store files because the user had not modified the folder (display) attributes on their mac, which was then added to the known-infringing list, and which then created this mess.
But it's valid to question whether Google has to scan all files for known-infringing files in the first place. That's really where it gets tricky, legally. On the surface, they absolutely do NOT have to perform such scans under the DMCA.
But then there are provisions in 17 US § 512 (aka the DMCA law) that state for example:
"A service provider shall not be liable [...], if the service provider
(i) does not have actual knowledge that the material or an activity using the material on the system or network is infringing;
(ii) in the absence of such actual knowledge, is not aware of facts or circumstances from which infringing activity is apparent; or
(iii) upon obtaining such knowledge or awareness, acts expeditiously to remove, or disable access to, the material;"
This is very vague. It wouldn't be hard to imagine that some lawyers could show up claiming that because Google received a valid takedown notification for a specific file known to be a "pirate" release of some movie, that Google should have known or at least "been aware of facts or circumstances" that all copies of the same file in whatever user accounts are infringing (which might not be the case thanks to fair use, but lawyers and most juries would not care). If they could furthermore demonstrate that Google does already have knowledge required to locate each and every copy of a file in Google Drive accounts easily (e.g. find out through discovery that Google Drive "deduplicates" storage), then it would be game over with most juries and Google's safe harbor in the case gets denied and they are found liable.
And that's only the US (DMCA) aspect of it. The German Bundesgerichtshof (Federal Court of Justice, highest court of ordinary justice) for example has found in the past[0] that service providers can be liable if they have been previously informed about copyright infringement and did not take "reasonable" steps to prevent further infringement, and that these "reasonable" steps may specifically include checking new uploads and existing files against a list of known-infringing files (or hashes thereof).
Yeah, it sucks that Google and other service providers scan files that way, even if these files are never shared, and it sucks even more when somebody makes a mistake and puts a benign files on the known-infringing list (which is something Google should then correct, apologize for and reset any account flag/reinstate any banned accounts that got in the crossfire due to Google's mistake), but I can also appreciate that law makers and courts around the world have put Google into a situation where Google defacto (if not dejure) has to perform such scans to avoid liability.
[0] In a case involving where Atari sued then-filehoster rapidshare over "Alone in the Dark", in 2012.