Because the Cap. Hill branch has a facade that incorporates ivy and plants into slats of metal, combined with a metal walkway. Inside, it's acceptable to snuggle into a window ledge and read. In fact, they have rules posted about what you can and cannot do while reclining in that fashion.
The drawback is that although it's great architecturally, it's small. The books are great for the size, but it would be awesome if it could move to a bigger space sometime in the future.