Taxes in the game appear to be a measure of the wealth of a state. The wealth of the state is of course benefited through institutionalized slavery. Why else do you suspect the Romans exercised slavery throughout their lands? If you can force other people(s) to labour for the benefit either of your citizens or the state directly, then you will generate more goods and wealth.
It seems you are arguing that slaves don't generate wealth because mostly wealthy Romans owned slaves. But of course wealthy Romans' wealth benefited the Roman state as well through taxes.
Basically, if you were to model this, you would be considering how much free labour exists to benefit the state directly (slavery to the state) or indirectly (slavery to citizens). Surely you must see the logic in reasoning that somewhere with more slaves will create more wealth for the state either directly, through forced labour, or indirectly, through the taxing of the outcome of forced labour?
