But to be able to make that distinction, you have to be enough of a gamer to understand that a game is defined more by its mechanics than its screenshots. Moreover, you have to be a gamer who has played Minecraft. To anyone else, Digcraft looks new and Steamtown could be mistaken for the rip-off.
Consider how this would work in a world where companies could sue other companies for making "similar" games. Remember that juries are ostensibly drawn from the populace in general. When thinking about filling out a jury, consider the following:
In a randomly selected group of twelve people, odds are strongly against even one person like you or I landing on a jury. An overwhelming majority of your peers are going to be people that have no frame of reference for judging between Steamtown and Digcraft. A good lawyer can convince those people that Steamtown is a ripoff of Minecraft. A good lawyer could convince a similar jury that Digcraft is not a ripoff of Minecraft.
Any law that requires a jury of informed, engaged gamers to reliably arrive at a sensible decision is doomed to fail, because there aren't enough of us to make that happen. I'm not saying the courts would always get it wrong, but there would be a lot of room for legal shenanigans. In the end, the winner would not be decided by who is right, but who can fight. And we know how that usually turns out.
If there was a law to fight against people like Zynga, it could easily be abused by big publishers to stomp out the competition. Can you imagine how hard it would be to make the average Angry Birds player understand the difference between Mass Effect and Halo? The difference between Quake 4 and Prey? Fallout 3 and STALKER? Why Torchlight is a "Diablo clone" and Neverwinter Nights 2 isn't?
Can you imagine how much EA would love to have the power to drag the developers of Anno 2070 or Tropico in front of a jury and claim that since these games feature overhead city-building, they're clearly Sim City ripoffs? Can you imagine how litigation-hungry Activision would behave with that kind of power?
My point is: Don't sweat this kind of stuff. Yes, Zynga is really, really annoying. They get rich selling obvious copies of things made by smarter, more creative people. It sucks, but other industries have to deal with this same problem. There are knock-off movies, knock-off sodas, knock-off cereal, knock-off clothes, and knock-offs of just about anything else we might consume. We've got laws to keep the bad guys from using the brand names of the good guys, and anything more might just be asking for a worse kind of trouble.
Shamus Young, once wrote a Tetris clone to teach himself C++. It was a lot of work, but he had the good sense not to ask money for it.