In what circumstances will you have 100%, unobstructed line of sight to:
1. Your car.
2. Your car's avenue of travel. [1]
[1] This is the important part. If you can't clearly see the entire path of travel, you could run a child over. Tesla obviously doesn't give a shit about that, because their collision detection doesn't work, and because the feature does not require this kind of line-of-sight to work.
The former is hard, but they want to be first to market, so they don't care about not building it right, and the latter would make this feature near-useless, so naturally, they don't ship this sort of fail-safe.
This is a critical part of the overall full self driving equation. Any vendor working on this problem will be spending inordinate amounts of resources on this as parking lots are one of the more challenging environments.
In certain locations (especially in winter) this will be a very nice feature. This is especially true to anyone with disabilities or extreme laziness. ;-).