You can only drag one card at a time right now. I think this is fine for the user story map backbone (i.e. journeys and steps), but when you want to move a collection of stories at once into a different release, this can be a pain as you have to drag them all one by one.
The main use case here is moving stories out from "Unplanned" and into a planned release. If you have ten stories, it would be great to move them all at once to a different release (with their parent steps unaffected).