Contrary to what many people think, dogs aren't doing this to cover up whatever waste they just left behind. In fact, dogs have scent glands in the pads of their feet, and by kicking back and scraping their paws along the ground, they are trying further spread their scent around and mark their territory. In dominant dogs, this could be a signal to other dogs to keep out unless they want trouble. In submissive dogs, this could be their way of letting other canines know that they've been there and they're not looking for a confrontation.
Some people don't recommend allowing dogs to carry on with such behavior, as not only does this create a mess, but it could also mean you're losing control of your pet, particularly dominant dogs. Dominant dogs who try to spread their scent aren't looking to you for direction, but actually asserting their leadership in the area that they've marked, which probably means they don't have much respect for you either. Even if your dog is mostly well-behaved, allowing such behavior basically encourages the pup to take control of a walk.
And a walk should mostly be for that purpose — walking. Walking or running outdoors should be more about exercise than anything else. Training your dog to eliminate outdoors should always be in a specific area, this way you can ensure you have a well-behaved pet that won't leave waste anywhere he/she wants and cause a mess that your neighbors will scowl at you for.