Disney probably design it intentionally to incentivize people to spend credit.
The Contraband Safehouse can be built at HQ 6.
The Droideka Sentinel Platform can be built at HQ 6 also, but requires 12,000 Contraband.
You acquire Contraband at HQ7, by building the Contraband Trade Port. (or purchase the Contraband using Crystals.)
I imagine, a lot of people are camping at HQ 6 because there is very few units/Transports to gain after HQ6. This is also an attempt to get more people to upgrade. Worked for me.
Here's a video on the dialog when you begin to build the Contraband Safehousehttp://youtu.be/xCwTP01cnjA