It's a tossup whether the WBC would have known about the rally and showed up on their own anyway. I think they're probably just going to get themselves noticed in the news for one, and secondly hoping that some evil atheist pushes them around so that they can play the victim card and sue them. I'm hoping all atheists who attend there will be civil since there's nothing to be gained by attacking the WBC. They've done far more to promote atheism than any atheist could ever hope to do.
Christian apologetics is the art of rolling a dog turd in sugar and selling it as a donut.