From what I've seen on Amazon, Barnes & Noble, and BookFinder, I suspect that the book is out of print, and those $100 + prices are being asked by private booksellers offering new or like-new previously owned copies (the price reflecting scarcity). In that case, the money would not be going to Macmillan.
The book is probably still under copyright, but if Macmillan is not keeping it in print, they might figure it was better to get anything at all from BW to add it (for approximately 0 effort on Macmillan's part) than to have it be completely unproductive. If it's still in print, though, that would be a different story.
David Rensberger
Atlanta, Georgia