It depends on whether they are there voluntarily in the first place or not - but also apparently people trapped into this situation very soon get a sort of Stockholm Syndrome where they become mentally dependent on their captors (as well as physically for their food etc).
If they are not being paid the 'going rate' for their work, and everything that they are 'paid' is then deducted for food & lodging then this is slavery, plain & simple, and is wrong. (From what I can gather, often the captor uses some trick like charging in advance for the lodging so they are always 'in debt' and can never earn enough to pay it off...)
It is the same with people who are trafficked into this country - they can never pay off the cost of getting them in and get trapped into slavery as a result.
It always irritates me when people say that slavery has been abolished. It hasn't. It has only been made illegal, which is very different. The UN produced a report a while back saying they believed there is more slavery in the world today than at the height of the slave trade.