I guess it kiiiinda makes sense, because citizens that are otherwise unemployed would probably be called in for big labor projects within cities, and more or less everything you produce basically represents those. Plus it gets across the point without seeming too confusing. Seeing a bunch of "unemployed citizens" in your cities makes it clear that, oh, they're not the usual laborers out in the field, and it would probably be better if they were put to bigger work. Calling them a "labor force" might get across the impression that you're supposed to have them there, or that they're some kind of other type of citizen, or something, but seeing that they're titled the unemployed gets across the point simply - they're citizens that are not working tiles or specialist buildings.
One must always keep in mind accessibility and marketing. It's not just the experts at Civ that play these games, it is also the people who are just kinda into it and think the game is cool but aren't super knowledgable at the numbers and whatnot. You don't want to confuse people that aren't very well-versed in the nature of the economy and historical labor forces. Sure that might be treating people like they're dumb a little bit, but it's best to keep things clear and simple in layman's terms whenever possible.