When the Fair Labor Standards Act of 1938 came into effect, employers became allowed to pay staff less than the minimum wage if they could make up the rest in tips. In some states, hospitality workers can still be paid as little as $2.13 (£1.60) per hour, with the assumption that tips will bring the wage to the federal minimum of $7.25 (£5.40). This has made tipping optional in theory but mandatory in practice — and some customers say it’s now reached tipping point.