The regulated sport of weight pulling is humane, yes. Anything can be inhumane when done improperly, but there are rules and guidelines in place to ensure the health, safety, and well-being of a dog when it is participating at a pulling event.
The important thing to realize is that you cannot force a dog to pull. Pulling is something the dog must do out of its own free will, and most will do it gladly with excitement and enthusiasm. (Just like your dog might do on a walk? eh?) Dogs, especially those bred to pull, need that outlet just as any other breed would benefit from doing what they were bred for.
It gets sticky when you have unconditioned dogs trying to pull more than they should. Just like you and I would need to prepare to lift weights or run a marathon, dogs are at risk for injury when their desire to pull is stronger than their muscles.
It is also fun to know that any breed can participate, so while you might have a certain image in mind, some championship winners have been the Norwegian Elkhound, Alaskan Malamute, Siberian Husky, Patterdale Terrier, Miniature Schnauzer, Basenji, Boston Terrier, & Chinese Crested!