How can businesses such as restaurants avoid paying the national minimum wage (NMW)?

Answer 1

This answer is for the US - I don't know where you are.

The minimum hourly wage of 5.15 does not apply to tipped employees.

"A tipped employee engages in an occupation in which he or she customarily and regularly receives more than $30 per month in tips. An employer of a tipped employee is only required to pay $2.13 per hour in direct wages if that amount combined with the tips received at least equals the federal minimum wage. If the employee’s tips combined with the employer’s direct wages of at least $2.13 per hour do not equal the federal minimum hourly wage, the employer must make up the difference. Many states, however, require higher direct wage amounts for tipped employees." from http://dol.gov/dol/topic/wages/wagestips.htm

Answer 2

Because tips are counted as part of a server's wages.