Obviously I can't possibly speak as to why they chose to do what they did. But I would assume that making a motion to dismiss due to the fact that arbitration has already been agreed to (seemingly unrelated from your perspective but from a legal perspective is really the only substantive aspect, so wildly related) is far less scandalous than making a motion to dismiss with no recourse for the plaintiff at all and would be far more damaging to their reputation.

And that DOES make sense.