Actually, copyright laws do not apply to gameplay. It applies to the passive stuff - the graphics, the sounds, the text, even the code - but not the interactive stuff. I guess it is possible that you can patent gameplay rules, but I don't think anybody has successfully done it or even defended it. I know Atari went after some clones of Asteroids a few years back, but I don't think that ever made it in front of a judge.
And I think this is a good thing. There has to be a line at which something is TOO similar, and I'm not sure there is a way to define that line with gameplay. If you add the Minecraft crafting system to Pacman, is that too similar to Pacman? Or Minecraft? How close is too close?
And is there a point where that line would defined that would make you uncomfortable? Have you see what is going on with software patents? Do we want that bogging down games too?
I'm not defending Zynga. I piss on Zynga. I've had
people take ideas from me and create 3DS games. I do not condone this practice at all, under any circumstance, but I disagree with it on a moral level, not a legal one.
Zynga are assholes, but truly, is this news to anybody?