Because they bring many units taken from other factions that are not available to their parent faction, which is enough for me to see them as apart. That plus the fact that the designers themselves have put them under NAA and not Nomads.
That's most of the sectorials. As for StarCo, all of the core sectorial troops, the mainline, is Nomad. Everyone else is various types of mercs, mostly from Ariadna and Haqq. This is done for lore reasons. Why also explicitly say StarCo is Nomad, it just pretends it isn't so it can do work for people who cannot do work with Nomads.
Per the interviews at the Interplanetario, the StarCo personnel are "reservists" that act as a detached, self-sustaining source of reinforcements should the Nomad Nation needs them (Mas allá de Mordor, see interview to Gutier or Carlos Torres, the very first two). Sure, they can run "deniable ops", but the main objective seems to be a "law abiding" professional mercenary group (thus being so detailed in Uprising) that also keeps a reserve of Nomad Troops ready to be deployed, but in a way that the Nomad Nation doesn't have to pay to keep at the ready.
