I am currently working on an article about copyright laws and how they apply to homeschoolers in the area of workbooks. In talking to several authors and publishers, I am shocked at the extent of the problem of illicit copying. I've heard stories of people returning books for refund with photocopies accidentally left inside the books, Catholic schools buying a few books and then photocopying for the rest of the class, support groups purchasing workbooks to be photocopied by everyone in the group, and more.
I'm sure that most of this is not done maliciously, but because they are not aware of the harm done by their actions. It's okay to make copies of workbooks if permission is given on the copyright page. You can also write to the publisher and ask permission. Often times, they will grant permission especially if you truly cannot afford to buy multiple workbooks.
Look to the Winter issue of Heart and Mind to read the article. In the meantime, I sure wouldn't mind hearing input on the topic.