Actually After reviewing some good references, I have not found any recommendations for the use of treadmill in gait training in THR postoperatively. The following sources were reviewed to assess whether or not there is any justification of Treadmill use postoperatively following THR.

Therapeutic Exercises, Foundation and Techniques, 4th Edition, By Kisner & Colby

Tidy's Physiotherapy, 13th edition, By Stuart Porter

Cash's Texbook of Orthopaedic and Rheumatology for Physiotherapists, 2nd Edition by Marian E.Tidswell.

Patient following THR requires a lot of safety precautions as soft tissue healing around hip takes 6 weeks and is called a period of Maximum protection while new bone growth takes several months. Joint position sense also develop over several months following THR.

not always nwb for 6 weeks and usually you can go straight to FWB after 6 weeks anyway
Moreover, in uncemented approach FWB is not recommended straight. If the surgeon has used a cementless implantation then the patient is non weight bearing (toe touch with elbow crutches - hopping is not safe) for 6 weeks, then partial weight bearing for atleast another 6 weeks. This allows for the growth of fibroblasts into the hydroxyapatite coating.