"I believe that was implemented so that others couldn't submit subscription requests using your email address."
Of course. That makes sense for stand-alone groups.
However, in the case of a subgroup, the person is already a member of the parent group -- which means that they have a trusted relationship with the owners of the parent group. Why make them jump through another hoop to join a subgroup?
Obviates one of the major potential advantages of having subgroups.