EDIT: something else occurs to me too. When you are presented with a list of words to draw, you have the option to "bomb" the list and get new words. As I mentioned, if you force quit the app at this point, and come back to that round, you will be offered different words, for free (definitely an exploit if you are patient enough to go through the process of force-quitting and restarting). I guess putting a back button on this screen would pretty much obviate the need for the "bomb" button, and would directly impact their bottom line since people wouldn't have as many opportunities to use their bombs. I have a suspicion that that's the reason there is no back button...the fact that it doesn't give players a chance to back out and keeps them a little more engaged it probably just a happy side effect.
Minor quibble, I know, but it does seem odd that you get new choices just by quitting when they really want you to pay for them.