They are protecting their brand, and are trying hard to create a strong trademarkable portfolio. Copyright expires, yes, but none of the case as I understand it really brought copyright infringement into question. Instead it was about trade dress and trademark. Those don't expire. Xio created all their own assets and material, and only reused ideas to make a game. People reuse ideas to make games all the time.

They're a smart business, and very tactical. I think when they do decide to drop the hammer on someone it's because they know they can win.

They also don't do it if they don't need to. If you beat them in a search engine ranking, you better believe they'll come knocking.