These historical women are not prostitutes per se, it is only in the eyes of western christian religious men that they see them as prostitution because the bible uses prostitution as having sex outside of the one and only wife. The practice of Islam of having more than one wife is prostitution or adultery in the eyes of the bible and hence why these european missionaries saw that.
At that time many noble men had illegally "wives" in different towns besides their legal wives. Some of them had 2, 3 or more "wives" also called kimit, who seats and waits until her secret "husband" comes. These women will get all the support they need from the noble men continuously as a regular wife but they are mostly not in the same town where his main residence and his wife lives. They also had farmland in different locations and these illegal wives "saw over" the land.
Because only rich nobles could afford this kind of life, the ladies also got good amount of support sent by their secret "husbands". You have to understand that at that time the soldiers and generals had to travel for days and weeks to reach a battle field in a very mountainous terrain. Can you imagine how many days it took to travel by foot from Addis/Shoa up to Adwa in Tigray to fight the Italians?
So most of them had their illegal "wives" all over the country. Though we were officially christians but in reality our men practiced islam in this regard and had their "wives" all over the towns

For christianity having sex outside of your one and only wife is prostitution that is how the bible calls it and that is how the christian missionaries called it. But for Ethiopians these were secret "wives" who got all the care and support.
Don't compare that to the modern prostitution business. Historical context is very important otherwise you will end up calling your second wife a prostitute