What truth? If they kick him out back in town, the only truth would be that they expelled someone that is less useful than a magic backpack.
Because these, like the majority of stories, are based on the culture of the writer and the culture of the intended audience.
Here both are Japan's culture. If you do not want to understand the culture, do not read things that rely on understanding it.
In Japan there is a stigma against firing someone, they bully people into quitting because quitting is better for their public image. Their work/employment culture is VERY different from the US.
They are dumping him in dungeon to hide that they fired someone, because firing someone in the middle of the city will look bad for them no questions asked, especially if the reason is not "you suck at your job" buit "we found an item that makes you redundant".
They are self centered, and dumping him in the way that costs them the least, as this means no severance pay, and no bad reputation.
This is not a stupid act, it is cruel and evil yes, but it is the option that from their perspective and based on the information they have at hand, is the most beneficial for them.
Everything centers around their reputation, because their goals require that the reputation meet criteria that they do not meet. A dead "teammate" better serves their reputation then a fired one does.