It is private property, but it is also a place of public accommodation, and the general public is invited, even encouraged, to enter. It is a close legal call, at first blush (I have not done any research on the issue.) Certainly the ladies could not be excluded because they are Christians (or belong to any other religious sect). Behavior, however, could be restricted. First Amendment rights restrict government interference, but not private property owners. Clearly they could not set up an altar and have a church service in the mall, at least not without permission. This is a much smaller act, but probably still something the owner could prohibit— LEGALLY.
How the customer base might react is another question.
IIRC (I can’t remember the case titles offhand, I’m not a lawyer), the SCOTUS at first ruled that because malls were private property, they could ban things like prayer and other religious activities. Then, it reversed itself and said that because malls are “public accomodations,” they can’t do so under the 1st amendment. Then, they reversed themselves again and basically threw the ball back to the states to decide instead.
I tend to agree with the side that says that because this is private property, they can *legally* do what they did. After all, if malls can be *forced* to allow prayer against their will, then who knows what else? Christian bakeries being forced to bake cakes for gay “weddings” or something? Oh wait.
However, I also think that what the mall did shows an incredible tin ear, and support people taking their dollars elsewhere. The mall can legally prevent people from praying on its property, but it can’t legally force them to spend money there. Maybe when enough revenue is lost and stores start to leave, the mall will clue in.