Using a solitary cuboid selector (x & dx, y & dy, or z & dz) with @e only selects players

Pretty straightforward: If you only use one volume selector in tandem with @e, only players will be selected.

Reproduction (through chatbar):
1: /summon ArmorStand ~ ~ ~

{CustomName:"Test"}

(Just as a test entity)
2: /testfor @e[y=0,dy=255]
Your name will be the only one which appears.