
If we're talking about DS1, all forced labor was wiped out when the penal colony planet Despayre was destroyed in DS1 weapon test. This was also to ensure the secrecy of the project (you never know when one of those scumbags escapes from the prison, going straight to the rebels to tell them DS1 construction details...

I would actually agree with canofhumdingers, that slaves were used only in the process of collecting/hauling raw materials needed for construction. Even DS2 was in a stage when only experienced specialists and qualified personel can finish the job.
But... remember this is just a movie! With two-sided comic book morals. Whole world is black or white: bad guys are super-evil-bombad, while good guys are next-to-a-saint-goodie-goodie. Nobody wonders about men conscripted to Imperial military service against their will (where would you get so many tallented rebel officers such like H.Solo, Carlist Riekan, Crix Madine, heck even Ackbar was Tarkin's pesonal aide/servant), who get shot on rebellion's path of wanton destruction... they simply didn't have the chance to defect to the good side... and rebel interdiction raids on Imperial shipping? Not all freighters are automated vessels, so think about rebel-wannabe pilots such as Biggs Darklighter blown up to chunks by proton torpedos fired by some happy X-Wing driver "on a holy mission"...


...but don't forget folks... that's what you get for... for being geeky!
