There are three possible reasons that can explain why doctors and scientists say masturbation is harmless.
Lying
The author believes there are scientists and doctors who know masturbation is harmful, but they lie about those harmful effects for political reasons.
Masturbation will make a human being weak and easy to control. By promulgating the false claims that masturbation is harmless, the lying doctors and scientists are actually encouraging people to become victims of their political conspiracy.
After the victims become masturbators because they believe it is harmless, even if they learn they are being controlled by others and want to resist they will have a very hard time doing so, because masturbation will have made them too physically and mentally weak to resist effectively.
Gullibility
The students who eventually become doctors and scientists are just like students everywhere. The trust in and believe what their instructors tell them.
When those instructors tell them lies about the harmlessness of masturbation, they accept whatever is said without question. They never consider the possibility they are being told lies for political reasons, so they never investigate what they are being told to see if it is actually true.
Incompetence
There are some scientists and doctors who deny masturbation is harmful because they either do not have the education or the mental capacity to comprehend how masturbation of the genitals at the groin can change other areas of the body.