It always was illegal, but hard for the IRS to prove so most got away with tax evasion. These days most people pay with a credit card which means there is an electronic record that the IRS can demand of the employer. With cash tips the waitress was collecting the cash themself and the manager didn't have a good way to get the correct number. If you report zero tips in the cash system someone will notice, but if you report everyone only gave you half what they really did there is no way to know.