But waiters, etc, they still have a weekly/monthly salary right?
Not in US. If the tips + the hourly salary do not meet the minimum wage, the employer has to pay the difference. e.g. in AZ, the minimum wage is about 8$/hr. The waiter may have a base salary of 3$/hr and after working for 8 hours and the tips are $40, he will get $64 with the employer covering the difference of $9. But if he has $50 tips, then he will get $24+$50=$65
Moreover, most of the low paying jobs here are paid by hours. Not weekly or monthly salary. Lots of employers keep the hours under 40 per week so they are part time employees, not full time, so the company does not need to provide lots of benefits like health insurance.