I think the mark is to let smoky know to kill Juliette. Once they get back to the big island I think he will be after her or something else will. The mark has to be punishment, if they were going to execute her and instead give her the mark it has to mean something bad for her. Going from execution to just a little mark that is hidden from view is too much of a step down. The mark is not anything of protection, that just does not make any sense, she is being punished for killing one of her own even though the one she killed was going to go against Ben's wishes.Why don't the others see that, she made sure that Ben's orders were followed. Kate and Sawyer were suppose to be let go.

Sorry but this doesn't make sense because Julliet had an encounter with smokie after her branding.... with Kate in the tree and he flashed lights at her and left her alone....