Another thing to remember: ever since the Spartan started their "Spartan" traditions, their population steadfastly diminished (despite turning women into baby factories). Lycurgus and others thought this eugenist policy would allow them to outperform Athens and vainquish this rival city, but the result was that Athens just had to wait a bit until the Spartans would be overwhelmed. Spartan laws are a way to fall on your own sword. So, unless orcish women are, biologically, much more fertile than human women (usually giving birth to entire litter of orclings rather than a single baby), this model is not sustainable. The tribe must have adopted this custom relatively recently (maybe after a disaster, thinking it befell them because they weren't worthy, or some similar scapegoatism).