Well, because often said snake oil can be quite toxic to your body and result in a lot of needless suffering. To be honest, most people know that if 'right to try' became a thing it would need to be followed up with 'you waive all rights to a lawsuit' because the moment an experimental treatment resulted in an extremely painful and accelerated death suddenly all of the people being advocates for right to try would sue them out of business.

In this scenario, it's a cancer expert well aware that these treatments could possibly result in the above and is willing to do so anyways.