> Why does Google Drive perform such scans? A lot of warez and piracy websites used Drive to share that content, but this kind of filter can be easily avoided by saving the files as an encrypted zip, rar, or 7z file with a password.

In my opinion, using this kind of filters to all the files you upload is pretty useless. The download traffic of a file or an account gives more information about some files used publicly than any other metric.