For all we know, the fanbox stuff could be their PL stuff they still maintained or sidegig that they've done alongside their current vtubing stuff so they can do stuff without restrictions or whatever. Can't really assume it as Niji's fault.
Like, the feeling that I got on the JP talents that isn't big ones is that, despite their numbers or popularity is on average without any sort of going up or whatever, they truly sees this vtubing stuff as sidegig rather than main job (like what most streamers do, making it full-time), so that they probably have so stuff to do outside the vtubing, maybe even have actual job that would probably giving them some safety net in case shit went down.
So yeah, don't account them NijiJP roomates making/owning fanbox as Niji giving them such rough time that they had to whore out, it's probably just them doing it for themselves or someshit. Maybe even doing things that Niji wouldn't let them do or the current place wouldn't allow (like ASMRs, for example).