alleged to be the work of Zynga -- Kill Screen has put together an interesting article about the legality of the practice and its effects on the industry. In short, cloning an entire game and stealing its assets is illegal. However, cloning a game's ideas is not -- see Scrabble vs. Words with Friends -- and, while it can be morally repugnant, it can also be healthy for the industry.
Game designers "borrow" from one another all the time, taking mechanics from successful games and implementing them elsewhere -- think stealth, cover-based shooting, power meters, triple jumps. That said, outright cloning is still a problem, one Kill Screen asserts we can address with education and a few notes from the fashion industry. In other words, teach consumers how to tell the difference between a knock-off and the real thing.