[ChrisW]

That's a good place to start Robt.
Speech, like all other action is motivated by desire.
Generally, I wouldn't speak to you unless I wanted to persuade you to do something or think something -- or at least consider something. (We speak to ourselves in order to give vent to feelings or to clarify our ideas, and I might speak to you for similar reasons without caring much whether you are listening, but that's certainly not the standard case.

But we generally recognize two sorts of "persuasion":

1. I persuade the store keeper to give me a carton of milk by giving him something he values more: $2.
or
I persuade my father to give me money for college by persuading him that this will best serve my long term interest (which he already cares about because he loves me). I take the money and use it for college.

2. I persuade the store keeper to give me the milk by persuading him (knowing all the time that I won't do it) that I will bring him the money tomorrow.
or
I persuade my father to give me money for college all the while intending to use the money for drugs.
or
I persuade others of my political view by stating true (but irrelevant) statistics, knowing that my hearers will misinterpret the statistics as supporting my view.

If you are the persuadee, you don't mind being persuaded in the first way, but you do object to being persuaded in the second way (as soon as you realize it). This is an intuitive or commonsense distinction that we might want to mark, however far we understand what's behind the distinction -- we might use "manipulation" to mark the latter form of persuasion. For the former, we might use the word "justification." But if you prefer to use 'manipulation' synonymously with 'persuasion', then we can find other words to mark the distinction.
A plausible view of the difference is that the latter form of persuasion gets you to produce an outcome purely for my reasons (because I don't give you a real reason, only an apparent one), while the former gets you to produce the outcome I desire for your own reasons. The former treats you as a tool; the latter respects you as a rational agent.

Whatever the explanation is, if we assume that all persuasion is really more or less like the cases in (2) we end up with a rather horrible view of human relations -- and of politics.
Legitimate government is a kind that can justify itself to the people governed -- tyranny may persuade the people to follow it on the basis of falsehoods (e.g., divine right of kings), but it cannot justify itself to its citizens. If we give up on separating the two kinds of persuasion, don't we give up on the distinction between tyranny and legitimate government?