What's their guilt?
They belong to a nation enemies of Rome: is this a guilt? They deserve to be sold naked like animals? to live for the entire life under a lash, nude or seminude, to be raped, to clean the house and work 18 hours a day?
And if they are thieves or poor people they deserve a destiny like this?
It's an horrific thing! They are innocent, anyway!!!
Slave girls were sold naked, then each master decided what to do with them: many were put in the mines, others in the fields; the most "luckily" became domestic slave girls: they worked topless or naked and often master ordered them to shave the pussy
However this scene was from a 21st century TV movie. Woman throughout history often shaved their pussy for sanitary reasons. As far as working topless or naked, the Romans actually had a large amount of decorum. Slaves were animals it's true, they looked at sex as a pleasure as well. But nudity was generally frowned upon. Even for a slave.
Nudity was not common in Rome, only in the bath-house or the arena, not on the street and certainly not in the house.