The danger with manipulation is that people can be lead to do immoral acts. History has many examples where people were manipulated toward wrong ends. The greatest example in modern memory must be Hitler's manipulation of the German people. People were coerced into lying, killing, stealing, and other recognized wrongs. Later, many people regretted doing this.
Psychological manipulation can be a very powerful tool.
So where does one draw the line. Even the thought of manipulating people can be repugnant to some. Others see the ends justifying the means. Different people have different moral values on what is right and wrong. So the place to draw the line would seem to differ from person to person.