It's probably just sloppy editing, but I *can* think of a reason to make the power an Immediate Reaction rather than a free action, although it's a corner case.
You can take any number of free actions. You can only take 1 immediate action each round. Assuming you had a way to get multiple uses of the item daily in a single round (okay, big assumption), you could immobilize more than one target with OAs.
Better wording would be to make it a free action, usable only once per round.
It does seem like this article wasn't vetted as well as some of the previous ones were - but I forgive it its flaws for all the sweet toys.