If you read as far as the second paragraph of the story then you would see that they are employed by a private firm contracted out by the Smithsonian to handle things like concessions and gift shops.
Reading further, the allegation is that the vendor is paying less than the minimum wage and not paying overtime that is accrued. I think that it's fair to insist that the Smithsonian investigate, which they seem to be doing. And if those complaints are true then the Smithsonian should cancel the contract and award it to another company that isn't going to rob its employees.
Pre coffee reading comprehension is not my strong suite.