Hi!
Why is social and military production treated differently?
That question should be aimed at the developers.
if you have sliders for military set to 50%, but a planet isn't building a ship, you still have to pay for military production on that planet?
Nope for a long time. If planet doesn't produce a ship, you don't pay for that. But if a planet is producing a ship, you always pay the FULL military production output. E.g. cost to finish a ship is 10 BC, planet's mil production is 100 MPs. You pay 100 BC, not 10. At least the latest DA 1.8 still works so.
And before you start asking why it is so, please notice that in first GalCiv versions (also DL!) you paid for everything, even it you didn't produce anything. I'm just gratefuly quiet for that what we have now.

BR, Iztok