Why does professor prohibit me talking to people? I'm considering a PhD and asked a professor at my university. I suggested a topic to him that he accepted. I applied and he offered me a position but I didn't accept yet.

Later he changed my project's topic to one that suits his interest. I said I would like to do the original topic that I got accepted with. Then I asked more about the project and it turns out he works in collaboration with another university. They divided the work up between themselves, but what the other university researchers is a lot closer to my personal interest than what he works on.

I asked my professor their contact details, so that I could ask them if I could work with the other university. He explicitly prohibited me from talking to them. He told me I would give bad reputation to him if I contacted them.

I am extremely confused as to why he prevents me talking to people. Even a simply inquiry email is not allowed. So I contacted the other university anyway. They told me that they would be happy to see me but they don't want to poach students from their colleagues.

I keep asking my professor if I could do my original topic with the other university that works on that exact topic that I'd like to do. But he keeps telling me that I either do his topic or I should do my PhD somewhere else.

I am utterly confused now and have no idea what is going on. Can someone please explain?
There are several possibilities for this behaviour. 


You're good, he knows that, and the collaboration with the other research group is more formal than you may know (maybe they are in the same funded project because it was the only way to be funded, while the two groups may be in competition). In this case, he doesn't want you to go “behind enemy lines”.
You're not the one he was looking for, he recently discovered that. He is very close with the other group and he don't know how to tell you that you are not a good fit for his group and relatives. So instead, he changed what he said in order to make it unacceptable for you, hoping that you will leave, and that you will also not go to the other group.
He's not reliable, he had a weird idea of what is a student.


By the way, except if you are a recognized genius, this is unlikely that the other team will accept you if they are close to your current advisor and want to stay that way.

In all three cases, only one option: run far away.