Why do people feel that it's "okay" to use cadaver bone for bone grafts?

Allograft: They have been shown over a long time, in millions of cases, to be safe and efficacious. The obvious advantage of using allografts (cadaver bone) is that you avoid donor site morbidity. In their words, less pain, less incisions, less risk of infection, etc. Your doctor can help you in an individual case decide which is best for you.

Allograft: The bone (or even tendons, fascia, etc) implanted that is cadaveric source is donated to a musculoskeletal tissue bank via organ donation. They are processed in such a way to prevent the passage of disease from donor to recipient. These tissues allow surgeons to safely replace damaged or missing tissue from injury or disease without having to harvest tissue from elsewhere on the patient.

IT is fine: Cadaver bone is often used in multiple bone grafting surgeries with great success. The huge benefit is that you do not need to suffer from pain in another area of your body from which your own bone graft is harvested.

Safe predictable: Allografts (from the same species) are safe and efficacious. The advantage of using allografts or donor bone is the omission of a second surgical site, resulting in less morbidity. The literature is replete with high success rates and very low risks.