I mostly disagree there too (at least in the US). Tipping is so ingrained in American culture that I don't think you can put the burden on the tipper to know that doing so can get someone in trouble with their employer.

Just as it's their responsibility to report the income, it's their responsibility to inform the tipper that they cannot accept and, if they want to explain, tell them that it could get them in trouble.

Persistence with that knowledge is unacceptable but that's another matter.