Their contracts are either pegged to minimum wage laws, or it is the basis for their negotiations on salaries.
Many union contracts for hourly wage employees, have clauses in them that guarantee that the lowest rung of the union pay scale be at MW + some amount. So when minimum rises, so does everyone in the union because the bottom of their ladder goes up.
$15 minimum wage would result in a $8 per hour raise for any union employee in this situation