Slaves don't vote, and slaves don't even get paid much. Anyone can get paid, but slaves get paid almost nothing, and they can't survive on it. Most slaves don't really have a choice in the matter. Sure, there's employers, but typically the terms are dealt and agreed upon with some level of transparency. Slaves are kidnapped and sold. You trying to make it seem anyone is a slave if they vote is just a massive 'f**k you' to slaves who are taken away. It devalues the horror of slavery, instead of spreading the issue. Slaves are mistreated. They don't pick masters, masters pick them. And if someone became a slave, they were probably deceived into becoming so, promised something better but aren't given it.
And yes, back in Roman times, slaves can be treated fairly, but this was a rare occurrence.
Anyways, the only time I'd believe you is when I have a shock collar around my neck, and a guy with a whip torturing me for dropping his drink.