They can do that because unions are part of the labour market structure, and can do collective bargaining directly with the employers, instead of leaving the government to impose a one-size fits all minimum wage [0].

https://www.fastcompany.com/3052798/can-fast-food-work-ever-...

That to me is the right way to approach this (advocate for workers' interests, but leave the government out of it), but if that was ever considered in the US, a good chunk of the HN readership would go apoplectic and beg for the state to intervene.