The minimum wage for certain employees is lower (by several dollars) than the usual minimum wage so although you are not the employer, you are expected to pay part of the worker’s wages directly, not just indirectly as customers do in most industries.
From the NYS Department of Labor:
Some industries make allowances for tips; thus they set a lower hourly rate. For example, food service workers may earn $4.65 per hour because their total compensation includes expected tips.