"Compress item" inside Time Machine backup Trying to compress (by right-click + "Compress item") an item inside a Time Machine drive's /Volumes/drive_name/mac_name/Latest folder will put the resulting .zip file on /Users/user_name/Desktop on the host computer.

This of course does not happen with other external drives not used as Time Machine backups.

Why does this happen? And how does it work, under the hood, the process of detecting that I'm trying to create a new compressed file in a Time Machine drive and of making a copy of the new compressed item to ~/Desktop?
Why does this happen?


Because the Time Machine backup is read-only. 

This behavior isn't specific to Time Machine; the same thing will happen if you try to compress an item that's in a read-only area of your hard disk (like /System), or an item on a read-only disk or network volume.