Not all airlines use the infant seat belt. There is an open debate over whether they are effective or not. Different studies show different data, so it depends on the airline which one they want to believe. I´ve recently read an article based on a study (based on research) claiming that infant seat belts can do more damage than anything. This because when a pax holding a baby fastened with an ISB is thrown forward with the impact, it will crush the little thing causing more damage than the impact would have.
There are EU carriers (I believe one is LH but correct me if I´m wrong) that don´t use ISB basing this decision on this kind of studies.