True, the United States has a "first to invent" patent rule, but the government's definition of "invent" is bit tricky. The court precedents say an invention isn't really "invented" until the inventor either files a patent, practices the invention (that is, makes or sells something that uses the invention), or publishes a detailed description of it.
To oversimplfy that a bit: It's not legally an invention until the public knows about it. Inventing something in secret doesn't secure the inventor's rights.
And, as you point out, the WOTC patent was filed at least 5 months before WizKids demonstrated "Pirates" at a trade show. If WizKids wants to say they invented CSGs (which doesn't seem to be their argument, by the way), they need to prove that there was a public record of the invention (by anyone who isn't WOTC) before the patent was filed.
At the moment, WOTC has the prima facie evidence of invention, because they were the first to prove they were trying to invent something. Which sounds crazy when I phrase it like that, but that's the way it works.