That is correct, but putting a hurdle like this into place will stop the majority of people that don't know what they are doing and are just going to upload your sensible data to all sorts of places.
We're talking about Google Docs. Ctrl-A Ctrl-C takes moments, even in read-only mode. Sure, some hypothetical technology may be able to prevent this, but that's just not how people share documents online right now.
Most people know how to screenshot or take a picture.
Similarly, these kinds of measures ("it'll make it harder which is better than nothing") usually just ends up annoying legitimate users while not providing any actual security.