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I agree with you 100% about Zynga. I'm sure they would disagree, but when I look at that company I see a business that doesn't focus on original ideas, but rather on monetizing the ideas of others. To use your example of Harvest Moon, Yasuhiro Wada brought a new concept to gaming. I mean, really, who else would have thought of a game where you struggled to grow turnips for hour after hour as the core of your experience, then actually made it amazingly fun. Not Mark Pincus, I'd wager. But what Pincus did think of was how to use social media to make a fortune off of Yasuhiro-san's concept.
You mentioned copyright law. Well, Zynga may not be a company I respect, but I do acknowledge that they understand how to minimize risk of a copyright violation. I'd wager that there is not one line of code that is identical between Harvest Moon (any of the series) and Farmville. That alone makes it much harder to prove infringement. I'm sure that they would point to other games that are based around resource management to claim that Harvest Moon really wasn't an original concept, even though I think it was too. EA has the resources to spend to litigate its claims aggressively, but there is no way the Zynga hasn't recognized the high probability of litigation under its business model and planned accordingly, from development to programming.
To be fair, gaming has been a fertile ground for taking and refining existing games for decades now. If you are old enough, think back to the early 80s, and how many space invader clones there were on game systems. Or how many Bejeweled clones are out there now on Google Play or the App Store.