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.