Why can't I set an Expression for an entity?

Lets say I created a new entity annotation under on the participant level called "related_participants"

Now I want to run a task on "related_participants" for a participant

I would write:
this.related_participants in the expression when submitting a job

I cannot do this because:

Define Expression
Disabled - selected entity is of root entity type

Is it possible to disable this behavior with a checkbox?

This isn't a huge deal because I can just use the api.