I'd rather not turn this into a debate about this subject, so this will really be my last post regarding it, and you're free to have the last word after that, but...
You forget that militaries of old were used to keep law and order in newly conquered territories, and while oppressive, this is certainly a form of law keeping. Another thing to keep in mind is that, aside from like the Romans and a few other empires, most ancient militaries weren't proper standing militaries, and generally drew on drafts, local militias, and whatever retinues came with the nobility and knights for their armies due to the fact that they just didn't have the resources for them, and thus come from the same pool of people that manned the town watch or city guards.
I will admit, SL2 as a setting is a bit different from standard medieval society, seeing as magic and other advancements exist, but at the same time, most of the nations in SL2 are nation-states rather than full blown empires with a massive pool of resources to draw upon for something like a standing military let alone the unified government, such as Karaten and Chaturanga. Aside from the Empire, Alstalsia is really the only other nation to "own" their entire continent, and even then most of it is uninhabitable due to basically being fantasy-Australia. Arguably, out of every single nation, only the Empire would have the resources required to own a real standing army, and if the other nations have nothing of the sort that would necessitate such a dedication of resources, well... It's feasible the Empire wouldn't care to devote that to it. You could argue that it's more of a case of available resources as a reason for why guards and militaries are one and the same in the setting.
As for your comment on the "kinds of people" who would join... I mean, in absence of a local police force, anyone looking to serve their community would likely just join the alternative, that being local militias, or the army... Since the army is the police. It's not like all those people who want to protect their communities just decide to not because there is no separation of power between police and military, they'll likely just do it anyways if they're really patriots of that sort. It's not like they'd just treat their own countrymen brutally or something, and pillage and rape the cities they're there to guard. I mean, they generally come from these places. These are their homes. Just because you put on a soldier's uniform, doesn't suddenly turn you into a violent barbarian that would brutalize your neighbors and your family.
You forget that militaries of old were used to keep law and order in newly conquered territories, and while oppressive, this is certainly a form of law keeping. Another thing to keep in mind is that, aside from like the Romans and a few other empires, most ancient militaries weren't proper standing militaries, and generally drew on drafts, local militias, and whatever retinues came with the nobility and knights for their armies due to the fact that they just didn't have the resources for them, and thus come from the same pool of people that manned the town watch or city guards.
I will admit, SL2 as a setting is a bit different from standard medieval society, seeing as magic and other advancements exist, but at the same time, most of the nations in SL2 are nation-states rather than full blown empires with a massive pool of resources to draw upon for something like a standing military let alone the unified government, such as Karaten and Chaturanga. Aside from the Empire, Alstalsia is really the only other nation to "own" their entire continent, and even then most of it is uninhabitable due to basically being fantasy-Australia. Arguably, out of every single nation, only the Empire would have the resources required to own a real standing army, and if the other nations have nothing of the sort that would necessitate such a dedication of resources, well... It's feasible the Empire wouldn't care to devote that to it. You could argue that it's more of a case of available resources as a reason for why guards and militaries are one and the same in the setting.
As for your comment on the "kinds of people" who would join... I mean, in absence of a local police force, anyone looking to serve their community would likely just join the alternative, that being local militias, or the army... Since the army is the police. It's not like all those people who want to protect their communities just decide to not because there is no separation of power between police and military, they'll likely just do it anyways if they're really patriots of that sort. It's not like they'd just treat their own countrymen brutally or something, and pillage and rape the cities they're there to guard. I mean, they generally come from these places. These are their homes. Just because you put on a soldier's uniform, doesn't suddenly turn you into a violent barbarian that would brutalize your neighbors and your family.