As already mentioned, permissive parents are inconsistent. Over time, children learn to manipulate parents to get their way. Call it ego-centricism or selfish behavior, children do manipulate. Because they can and because permissive parents let them. I would go so far as to say that permissive parents actually reinforce their children to manipulate.
Children learn a false sense of control over adults that increases their manipulative behavior.
Inconsistent limit setting is a form of intermittent reinforcement scheduling, a fancy behavioral term for a very powerful reward system. Permissive parents also confuse aggressiveness and assertiveness. One of the negative consequences of permissive parenting, for parents, is that they end up feeling like slaves and martyrs to their children. They resent being taken advantage of and never being shown respect or appreciation for all they do for their children. Setting inconsistent limits is a symptom of inability to set firm boundaries for the self.