I'm not aware of a specific argument against doing the UDT thing here. It's just a combination of the UDT-like conclusion being counterintuitive, UDT being possibly wrong in general, and the fact that we don't really know what the UDT math says if we apply it to humans or human-like agents (and actually we don't even know what the UDT math is, since logical uncertainty isn't solved yet and we need that to plug into UDT).