I think the question of whether any building must have a population requirement should come down to game balance.
We need reasons to move colonists off our homeworld. Now, true, many reasons already exist. But buildings population requirements are one of those. The higher the pop requirement, the greater the incentive to move population to faroff places.
If Sector Commands have no pop requirements, they can be slapped down on any exposed piece of rock. That allows them on to be hidden or placed in systems with no viable colonies. This forms more the idea of a self-contained command post for the sector. Like a government bunker or something.
If they have pop requirements, the functions become more colony-centric. High population planets become more valuable as potential capitals to host these facilities. Now they are more like parliament or the Pentagon, requiring a large population to support them. This provides impetus to form little nexuses of government rule on populated planets throughout your empire.
When planets had no pop limits, the first option made more sense. You can dump enough infrastructure on any barren rock to run the Sector Command, so why force players to jump through the hoop? Now that there are maximum populations and most asteroids may struggle to be able to house sufficient population to run a Sector Command, perhaps the second may add more dynamic gameplay, encourage more desired behaviour and make more sense.