Modern? It's a trick that's probably as old as discrimination itself. Here's a rare case of an American judge speaking openly about this - the quote is from a 1941 Florida Supreme Court opinion (Watson v. Stone):

"I know something of the history of this legislation. The original Act of 1893 was passed when there was a great influx of negro laborers in this State drawn here for the purpose of working in turpentine and lumber camps. ... It is a safe guess to assume that more than 80% of the white men living in the rural sections of Florida have violated this statute. ... There has never been, within my knowledge, any effort to enforce the provisions of this statute as to white people, because it has been generally conceded to be in contravention of the Constitution and non-enforceable if contested."

Note, by the way, that this also references the other common trick - you can also make rules that are seemingly not arbitrary, but are easy enough to challenge for those with the right knowledge and/or means.