Child object's collider (on a different layer) is interfering with parent Physics...

Here is a glimpse of my Hierarchy:

 Ball object (with sphere collider, rigidbody)[on 'default' layer]
      - Cube object (Cube collider) [on 'testLayer' layer]

When the cube object described above is inactive, my ball object interacts normally with surroundings (it rolls, an bounces predictably according to "real world physics").

When it's activated, the ball stops spinning, as if the square collider is hitting other colliders.

This is surprising, considering that in Project settings -> Physics, I made sure to uncheck everything related to 'testLayer' (so as to have it not collide with anything at all ever, including itself).

Why is this happenning? Does the internal physics engine have a problem with children on different layers?

The problem is solved by changing that cube collider to a trigger... Why isn't just disabling collisions in physics settings sufficient to prevent this from being an issue?

Thank you!