No one’s preventing properly designed and carefully run clinical trialS of COVID-19 treatments using HCQ. What doctors shouldn’t do is administer potentially life threatening drugs to sick patients based on rumor and speculation without any consideration. Such doctors are basically running badly designed, unsanctioned, unsafe experiments on people who’s ability to consent to the experimentation is compromised in any one of a number of ways, like being sedated and on a ventilator. Institutional review boards exist for a reason.

https://en.wikipedia.org/wiki/Institutional_review_board