You can demand things from the AI.

The problem is that you have to be so much stronger than the AI in order to make such a one-sided deal that it would probably literally be faster just to conquer the rest of their planets than it would be to make the deal.

So you can demand but it's more or less irrelevant by the time you can make a demand, the AI will surrender to the strongest opponent on the map that they have good relationships with before it gets to that point.

That's my experience at least. If the AI or minor AI is starting the conversation with 'Please, don't hurt me.' then they are afraid of you, whether they are on good terms or not, if they have no non-aggression pact.

It's something that does need to be tweaked, but it's probably better left as-is until the economy is overhauled a bit and various technology branches aren't all but irrelevant...