For one, most people don't equate indentured servitude with slavery. We generally think of lifelong service when we say slavery, which isn't part of the penal system. The penal system also isn't generational and people aren't born into slavery. There's grounds to call it slavery, yes, but the context you're bringing it up in is in comparison to the African slave trade and you're diminishing the suffering those people went through by saying that what happened to them was just like what we do to prisoners today. What happened to them was much worse.