Why are they published like this? Why?

I finally finished Dragon Tattoo today, and was in the mood so went ahead and watched the Swedish movie from a few years ago. The movie did quite a good job of cutting down on the infodump sections of the book while still making it make sense, I thought.

So, anyway, I'm on to The Secret Circle, and I find these books extremely annoying because they're a trilogy but published in two volumes, with a book and a half in each. They cut the second book off in the middle! It's maddening! That's why I'm reading all three back to back, because I can't just put it down when the second book is halfway through but the physical book I'm reading is over. I'm sure they did it for marketing reasons, but it's driving me nuts.