It's an error on the part of the dev (or an oversight). At this stage, I'd imagine they're leaving it alone in case trying to remove it breaks something.
I appreciate that, wow I thought it lead to something like maybe having a heroine as a henchwoman doing heists for you and corrupting them to renounce being a hero.(I mean it could be a placeholder as below the second unlock option, theres two red circles which indicates that maybe they were planning to have an upgrade option to the third tier and thats just a placeholder? I get that the dev wants to finish the events, but I think after the dev is finished with the events and finished with whats missing. Maybe introduce that third tier of heroine corruption? Sorry I keep saying it like I'm trying to manifest it to happen and theres so much you can do, it being there is triggering my OCD, its like an unopened present that cant be opened as I want to know what they wanted to do with it. Its probably like you said an error, but I see it as an opportunity to flesh out the corruption a bit more. (Hell you could even introduce pregnancy in the third tier as well as making them your henchwomen)
What would you want to see the dev implement in this game in terms of potential heroines or potential ideas?