Progressives have made the Fight for $15 a national issue. But Washington, D.C.’s Democratic city councillors this month moved to overturn a mandatory minimum wage for tipped workers after bartenders, waiters and restaurant managers served up a lesson in economics.

On Oct. 2 the D.C. City Council voted 8-5 to repeal Initiative 77, a ballot measure imposing a $15 minimum wage for all tipped workers by 2026. The wage hike was billed as a way to give workers financial stability and protect them from sexual harassment by restaurant...