Wasn't tipping primarily exempt from the minimum wage because FDR had to get the support of the entire democratic party to get his reforms passed, and the southern Dems refused to vote for anything that would make them have to pay black people. So tipping and agricultural labour was exempt from minimum wage laws, because otherwise the new deal would have gotten blocked by the presidents own party. And FDR figured that there wasn't enough of those labourers to kickstart a revolution by themselves, so he agreed to it.