This is madness!
First, I can't believe you are deleting our files without being really, really sure that we want them to be deleted. There should be some kind of 'this will remove this directory from your local drive...are you sure?' message.
Second, as the others have done, I also interpreted the option to 'Unsync' to mean that the local files would no longer be synchronised to the cloud, not that they would no longer exist on my local drive. If you had a confirmation message it would at least raise alarm bells, and in my case I would have cancelled the operation and double-checked what the command really does.
Third, this is not just about saving space; I would imagine that's of minor importance to most people. In my case I was trying to set Insync up to ignore my node_modules directories in my Nodejs projects. Those directories often contain platform-specific code, and since I'm mirroring between Ubuntu and Mac OS it's more trouble than it's worth to copy those sub-directories. I thought it would be a simple case of setting the ignore list to exclude 'node_modules' followed by doing an 'Unsync'.
But I was wrong, and now I'm sitting waiting (an hour or so now) for the sync to finish before I can restore the files. All very embarrassing when working in a client's office!
By the way, I really like Insync.