cannot negate user role condition for block visibility

The block module disables the UserRole condition's ability to be negated. In our business use cases, we wish to specify content blocks which are only visible for users WITHOUT a specific role. It appears that we can re-enable this functionality by simply removing the override and allowing the ability to negate the UserRole condition. It would be great to know why this ability is disabled in Drupal core, or to re-enable this functionality.