That statement isn't even about legal slavery ( which to be clear, you are absolutely correct about) it's more akin to why it costs 16 bucks to get a warm bud light at a ball game. Because, what other option do you have at the ball park?

Now, treating prisoners in the same manner is loathsome and immoral. The amount these outside groups charge prisoners for the ability to stay in contact with family is parasitic in just the worst ways imaginable. And, it speaks to the lack of humanity the owners of these contract companies have, as well as the lack of humanity those running the facilities have.