If you're concerned about the safety, the explanation on the page (which you should visit) is as follows:
For those concerned about the treadmill...A) There is an adult human in the room. B) Physical therapy for all animals can be tough to do AND watch; I oughta know! The dog may have stiff legs due to being trapped in a crate or on a log chain for who knows how long! This STRETCHES and increases muscle strength, balance and coordination! C) Walking in any way, shape or form is good for both body AND mind. The leash is to PROTECT the dog in a variety of ways. D) Puppies learn by example and mistakes. This appears to be an excellent quality, well built machine. It may not be a good idea to let it go until it gets hurt, but the pup was having fun and learning while being observed. I doubt it went on for hours! E) All the animals pictures appear to be really well kept; coats are nice, weight is about perfect and attitudes, STELLAR! Thanks for sharing!