Actually, neither the stance nor the attack has the Weapon keyword. The attack has a range of Melee weapon, which isn't the same thing. It's definitely an error, you can't have [W]s without the Weapon keyword.It's a stance that allows you to make a special attack, for the rest of the encounter. The attack does have the weapon keyword. The stance doesn't need it, since entering the stance does not involve any attack or damage rolls.
Actually, whether the power has the weapon keyword or not is pretty much in limbo right now: it was originally printed without it, but was added later with an update to the Character Builder / Compendium. However, said update never made it into any offficial errata document.Actually, neither the stance nor the attack has the Weapon keyword. The attack has a range of Melee weapon, which isn't the same thing. It's definitely an error, you can't have [W]s without the Weapon keyword.