It's insulting to call them workers. To imply that they willingly picked cotton and worked on plantations akin to paid help is ridiculous.
It's very upsetting to me because black kids learning from this book will think that their ancestors were treated fairly AND were willing. I don't think the world slave was redundant just because they said it was the slave trade in the same sentence, if anything it's only your white guilt that's trying to make you feel better about yourself by denoting them as 'workers' instead of what they were.