Why didn't the Doctor and River do this at the end of "The Angels Take Manhatten"? (SPOILERS)

At the end of this episode,

What I am confused about is, if the author of this book had prior knowledge of what would happen when they wrote it (as seems apparent early in the episode), then why

There seems to be another possible solution as well:

Also, for the record I'm only at that episode in the series, so there could very well be something in future episodes that explains it (in which case please refrain from giving it away :) haha)