I feel like its an exploit to surface your t1 subs on your naval factories early game as there's no way to counter it because when you push with any type of early navy all they do is hit the factories, not the subs so in theory 5 t1 subs with an acu and engis healing the factory could hold against 15 subs attacking.
I have no idea if this is meant to be a feature or if it's an exploit, I just thought I'd bring it to people's attention.