IANAL, and yes, you can not copyright game mechanics or ideas. This is why so many clones of every game exists. Because no one owns the rights to "a tower defense".
The successful suit was due to the playing field having the exact same dimensions as well as very similar visual expressions as regular Tetris outside of the just the game mechanic.
[1] http://www.wired.co.uk/news/archive/2012-06/20/tetris-clone-...