JQL for Unassigned Issues Makes No Sense

I just tried to find all unassigned issues in a project (by using the JQL statement "assignee = unassigned"), and got a warning that said: "The value 'unassigned' does not exist for the field 'assignee'"

Fortunately, the query returned the issues I was looking for. But still, why doesn't that query work?

I learned that the correct query for unassigned issues is: "assignee is EMPTY" How is anyone supposed to figure that out? Documentation is not the answer. Why can't we use the = operator and 'unassigned' instead of 'is' and 'empty'?