In my opininon riding on sidewalks is OK on main streets with high traffic and without cycle lanes for children and ladies. Otherwise bicycle is a vehicle, it has to be operated on roads. Sometimes I see MTB or BMX riders on sidewalks doing some bunny hops and stunt jumps when slalloming behind pedestrians, to do them high riding skill level is needed, they can cope with traffic with that riding skill so doing stunt rides on sidewalks is completely dangerous and rude.