Pure Speculation. Stronghold state systems could be OPEN ONLY.
If that was the case it would incentivise the community to either expand Stronghold systems, or work against them and prevent their spread. Similar to how you can fight back against the Thargoids, but with the added bonus that in Powerplays case you can also strengthen the power.
I doubt they'll make it open-only. Best way to stimulate open play is by giving extra rewards in open.