They actually don't state this directly - it's states with collective bargaining vs. no collective bargaining. You can be a unionized teacher in a state without bargaining rights.
Which just muddies the waters because if collective bargaining is banned then the unions are toothless. The correct comparison is collective bargaining legal vs illegal.
In my state the "teacher's union" is basically liability insurance for stuff that happens in the classroom, they have no real collective bargaining power.