The idea/notion "Masturbation is therapeutic" in Vinaya was highly personal experience. In Vinaya, this particular monk is not pleased to be in life of monk and had thin body and he got normal body weight after having regular masturbation. In this very personal experience the monk had tendency to do sexual intercourse but he cannot. By doing masturbation he was trying to satisfy himself but actually he is encouraging the fetter to overcome him. There was nothing therapeutic effect in any extent.
I will try to answer your question with medical example. While we are trying to test the effect of placebo medicine to sampled/surveyed patients, it show significant positive effect on targeted patients. But not a single FDA from any country will approved that it will have therapeutic effect on the particular type of patient we tested. Scarcely, placebo medicine will be commercially available for this particular disease. Masturbation is lightweight demonstration of sexual intercourse. If you cannot do the latter, you are satisfied with masturbation. It will make you psychologically feel better but is is not therapeutic cure to extinguish strong sexual desire (which is fetter in Buddha's teachings). It is just like placebo medicine and highly personal experience.
There is another example, if someone is highly allergic to nicotine and addicted to cigar, will change from cigar to cigarette be better for his allergy? So as masturbation, it will make your sexual desire to burn more inside (which is definitely fetter in Buddha's teachings). If you think masturbation has therapeutic effect, then you are changing from cigar to cigarette expecting same therapeutic effect of cutting off nicotine intake. And that is why masturbation is prohibited in Sanga society by Buddha.
Buddha said masturbation is not make people bored about Samsara (born, live, die, reborn, live, die cycles). It is not beginning of noble practices to stop/get out of sufferings (Samsara).
Masturbation has only placebo effect on sexual desire to intercourse it will make personal Raga to make stronger. Raga is one of three unwholesome roots and now (at the time of 2016 AD) people thinks masturbation is disgusting and not allowed to do in public (compared to smoking, people does not like it, but there are public smoking places).
People has mindset that sexual desire is normal to animal instinct and one should practice masturbation if he/she cannot find mating partner but clearly it is psychological placebo and not therapeutic solution to cure to make calm to sexual thirst because they do not know what is cause of Raga, what is supporting Raga to grow in mind, what is the nearest cause of Raga and how to get rid of it.