Just a hunch, but maybe you had the cell selected and tried to copy that, instead of selecting the text within the cell?

... that's not really better. Requiring the desktop installation of Drive for trivial cases like that means that the web-application isn't really a web-application, but rather a desktop application that uses the browser as a GUI.

Actually, that's due to Drive trying to support the system clipboard intelligently (e.g. such that if you pasted into MS Office, you'd actually get a table/cell structure, rather than just text). Browser APIs don't provide support for that level of copy-paste, so it has to be worked around.

Plain text copy and paste works fine without.