[philwelch]

Just because something is done surreptitiously doesn't mean it's against cultural norms--it may just mean the cultural norm is to be surreptitious about it. Cultural norms are weird that way.

For instance, maybe the taboo isn't against bribery per se, but against conspicuous displays of wealth--"flashing your cash". For instance, it's taboo to openly discuss one's salary or other financial details. You might think conspicuous consumption is a counterexample, but really it works for this point too--since there's no surefire markers of social class and it's taboo to just go around saying "ha ha, I earn 100 times more money than you", people who want to brag about how rich they are buy really expensive things and make sure everyone notices them. (Similarly, since there's a taboo about exposing certain parts of your body, women who want to brag about how fantastic certain parts of their body are wear tight clothing and cleavagey tops.)

If this trick works universally among high-class restaurants in a certain culture (be it the US, New York City, or whatever), it's indeed a cultural norm to do it--even if the norm requires surreptitiousness.