Well, it's kind of smart that way. Why accept any terms you give them when they're winning, and can force their own terms later?
The way peace acceptance works is that a country has War Enthusiasm and Warscore. The peace you have to offer them is their Warscore + their War Enthusiasm, so if their War Enthusiam is 10 and Warscore 50 they will accept an offer of 60 or above with things it wants. War Enthusiasm is why they're less willing to accept surrender if they hold their capital, because it gives them a boost to it.
Now, the problem as it is in release version is that the code that's meant to make the AI hard to fool is making it a bit too unreasonable so you pretty much end up having to wait for the AI to send you its demands, rather than being able to peace properly.
This is being reworked and a "What do you want?" button added to get rid of the frustration of being unable to sue for peace. You still won't be able to offer peace in certain cases where they're dead-set on annexing you or want to occupy more territory so they have more options, but the frustration of being half occupied and unable to sue for peace is something that needs to go.