"Whenever you have a copyright, you're protecting the expression, not the idea," Dallas attorney Mark Methenitis told Ars. "It's a line a lot of people have a very hard time with, especially when you get into games."
In other words, it's exceedingly hard to copyright an abstract game mechanic like "guessing five-letter words and giving hints based on correct letters." A game developer can file for a patent on an original gaming idea, a legal process that has been used to strangle video game clones in the past. But getting a patent is a long and arduous process that can fall apart if there's "prior art" predating the idea (or if the mechanic could be considered legally "obvious").