[rtkwe]

They can but doing so reduces their operational life time so they'll want to minimize the amount of the thrust budget they expend on slot filling launches. As someone else pointed out their prices are high enough to cover the cost of a reusable launch with maybe half the ports filled so they're probably just playing the numbers game assuming there won't be a large coordinated drop out of customers. And those prices don't include the main top port which will likely cover a fair portion of the operation cost of the launch by itself.