They dont need to say it, Mewtwo was just announced and you can get him for free. They didn't say its theonly way to get himby purchasing both games.
In a world of exclusive DLC - amongst other shitty anti-consumer game industry practices - I don't think it's unreasonable at all to believe that purchasing two copies of the game may be the sole manner of getting te character, even if it's not the safe bet either. You're upset with others for assuming that he can't be purchased individually, yet you're equally guilty of assuming that he will be.
The only facts we know of right now are that Mewtwo is coming, and people who have registered both copies of the game on their Club Nintendo account will get him. No other method of acquiring him was mentioned, notwithstanding that this would be a logical place to do so. It's probably just sloppy messaging, or a disingenuous way of getting people to rush out and buy both copies, but I don't believe it's fair to blast folks for grumbling at this stage.