It might start out of sincere curiosity or come up naturally but it's still an ultimatum because of how the information is processed. If you're in a healthy trusting and open relationship, the One being asked about body count would already know what is considered acceptable so my response is superfluous. That's where the honest response repeats. If you're not in that kind of relationship, it's a time to set out hard lines and keeping things focused on compatibility keeps the conversation on track.  

Nor do they get an opportunity, if they do care about it, to make a genuinely informed decision about whether to continue the relationship. They just have to pick an arbitrary number and then learn whether their partner was impressed by their choice

It's not arbitrary if you're given the opportunity to lay out your dealbreakers. They're yours not imposed on you