I think this is the spot where your logic stopped working.
The idea is that you don't care whether they agree. You'll listen to opposing arguments, of course, but the goal isn't to get them to agree. They're asking you because they're interested.
Or at least, that's how it should work. It stops working when it becomes an interview question.
The point is to see whether someone thinks originally. If you have opinions you're reluctant to share among peers without wording it carefully, and you've spent a lot of time thinking very carefully about the topic, then there's a pretty good chance you're an interesting person to know.
EDIT: Another way to phrase it: If you give an answer that you truly believe and that you've thought carefully about for a long time, and they disagree loudly and immediately, then there are two possibilities: Either they have given the topic more thought than you (which is entirely possible), or they are less open-minded than they think they are. Both outcomes give useful information to you.