Unfortunately, you've discovered the ugly face of textbook publishers and their greed magnified by the DRM capabilities of eBooks
While they've published new editions every year in the past, they couldn't force students to buy them, even if the prof listed them on the course book list (after all, the last couple of editions would be close enough for cash strapped students). But now, they can actually force students to buy a new textbook - at full price - every year.
This is not unique to enTourage, please understand. Hopefully as more students discover the obscenity of eTextbook licensing, pressure will come to bear on publishers to not time-limit their textbooks.