Oft times on small narrow streams, its the choice of the lesser of two evils: the risk of lining them with a cast from down stream, or announcing your presence from up stream. Easier to mask your presence from up stream when the trees and undergrowth are full of leaves. .
Its less of a concern in faster broken water like in the center of DF's 2nd photo, as opposed to the nice calm pool in the 3rd. The former will also usually have a surprising bit of depth to them. Less chance of the the fish seeing or being seen.