I would agree that their behaviour is inexplicable. The site prevents you from sending or retrieving more cash than their terms permit, or more frequently. So how can your activity be "excessive?"
Of course, they've given themselves a clever (perhaps) but customer-unfriendly (definitely) legal out, so there isn't much you can do.
As an aside, they have questioned my activity a couple of times, but have yet to pull the plug. I have cut back my transfers to approximatley half of their permitted allowance; we shall see how much longer it lasts.