Talent unions set minimum pay scales, not maximums.
Hollywood actors are part of a union (SAG). Do you think that Tom Cruise got paid the same as an extra that was also in the SAG for the last Mission Impossible?
I would very much like to know which broadcast or cable shows you think use non-union labor, because that would be a violation of almost all studios agreements with the unions.
AFAIK, only Netflix uses non-union labor on shows...and even they have entered into negotiations with multiple unions.
Union rules part of some union collective bargaining agreements are that everyone who works there has to be part of the union. Many states that do have right to work laws still force companies to apply union agreements to even those who are not part of the union.