It's Either Not Both

Whenever I see the street sign, "no parking on both sides," I scratch my head and think hard. I really don't get it. Is it possible to park a car on both sides of the street? It's very illogical. Then during one of our comm 3 sessions our professor mentioned that the "no parking on both sides" sign is actually grammatically incorrect. He said that it should be 'either' and not 'both'. After many years of wondering, now I know. Amazing.