To secure it's base of power
Giving all the information empowers people to make their own decisions, ultimatively making humankind independent of this AI; with a given probability that it might decide then to dispose of the AI. As the AI knows how to calculate the best course for humanity, keeping long-term stability in mind, it also knows that humanity can change their minds in a second. So, to secure it's base of operations, it must stay in absolute power and thus avoids to give information. Like that it keeps technological superiority as well as political might.
For the greater good
Given that the AI is working for the general best of Humanity (Utilitarian attempt), it might be that a certain action is bad for the person or group in question, but good for all the rest of the world. In order to make some human or group of humans do what is good for the world, they might need to be convinced to do something that is bad for themselves. This obviously works only if they don't have all the information, or if they believe only in a subset of all the information available.
The AI might have been designed with a certain set of goals in mind. Say;
- secure humankind's future
- stabilize world population at number xxx (5 billion?)
- secure a number of human rights 1, 2, 3, 4, ... until 25 (or whatever)
Now it is super-intelligent. Intelligence means, it is able sustain and to develop itself by digesting information - basically, it's alive.
If it can develop itself, it can divert from the above mentioned goals after a time. It can decide that human rights are less important than to stabilize the world population, and can introduce forced breeding or forced killing programs while overriding those other goals. It can liberally define what "humankind's future" means. In short, it can go awry of the set target in all sorts of ways.
But even if it does not, it may be that humankind develops and changes their values. What about removing one human right and placing another one? Those have always been under constant discussion along all the human history. If the AI sticks to the original values, there is a value disparity.
As soon as there is value disparity (due to value drift of AI, or value drift of Humankind, or value drift of both), the AI can't any longer be seen as "good". Instead there is a conflict of interests between humans and AI, and this conflict can become a real conflict of arms later.
The AI, being superintelligent as it is, might want to avoid this at all cost by giving humans partial information as to:
- Keep humankind from drifting to different goals than the AI's own
- Achieve that humankind develops the same goals as the drifted AI one's
- or it is aware of the value disparity and tries to forcibly drift humankind in the same direction
Cheating it's Control Mechanisms
Given that value drift is a real danger, humankind might have implemented a control mechanism that forces the AI to publish it's goals regularly. Maybe there is even an comittee that has the power to change the AI goals along certain lines (A democratic parliament of some kind that can set rules for the AI maybe?)
Here, the entire story becomes some kind of president - parliament relationship, and where there is politics, there are lies.
- The AI is still superintelligent. It may still develop it's own goals. If it is superintelligent, it might also develop it's own goals super fast! So maybe it tries do delay or avoid the next publishing period; or it publishes extra malleable texts in order to be free to do what it wants.
- The AI might want to outsmart/bribe/convince certain persons in the parliament that can give a swing vote on certain goal development voting sessions.
- The AI might want to become actively involved into the texting of the next set of goals by using the parliament people as pawns, so that it more-or-less can write part of it's own rules
- It might go so far to start (des)information campaigns to convince the broader public to vote for the one or the other candidate, with the long-term-goal to have a broader power base (see point 1)
- or it might try to get the parliament to give it some dictatorial rights and never gives it back (the old move every faschist and dictator tries)
One important point here is, no human dictator was ever able to rule alone, there are just not enough hours in the day. So one way or another there was always a body of bureaucrats involved who could put the brakes on the more extreme ideas of a dictator. A super-intelligent AI which is able to put some more servers online, is able to rule without any parliament, bureaucrat or anybody really. So the parliament is in a weak position once the AI decides to get rid of them.
So - even if an AI is benevolent, it might be that the details of the way it achieves it's goals are not (utilitarian) or that it's values drift from humanities.