A large percentage of people who work in the service industry are paid less than minimum wage because tips are considered a significant percentage of their income. This includes bell hops in hotels, folks who delivery pizza or Chinese food to your front door, folks who valet park your cars, manicurists, (most) hair stylists, coat check staff at nightclubs, (some) tour guides, 90% of the staff who work on a cruise ship, and of course waitstaff, bartenders, and sometimes even kitchen staff, who in some restaurants get a percentage of the waistaff tips.
If you can't afford to give a tip, then you can't afford the service. If the service was unsatisfactory, then you can, and should, refuse to tip. Otherwise, there is no excuse.
Some employers use the laws allowing them to pay less than minimum wage to staff getting tips as a way to get around paying staff minimum wage. In my neighborhood, a local Dunkin Donuts was paying its cashiers less than minimum wage, because there was a jar by the cash register that said "tips welcome." I went crazy when I heard this. Most people don't tip cashiers. But I think that most people DO tip waitstaff.
There was also a scandal a few years back at a local supermarket, which allowed "employees" to bag your groceries for you, but did not pay them anything. They had a tip can, and they were working only for tips.