It is not cruel if it can be shown that this restriction is in the patient’s own interest. The status quo prevents patients from living out their last days on a stream of experimental drugs. We prevent drug companies from using them as risk-free testing (under your policy drug companies would presumably be able to shrug off any responsibility for adverse consequences by saying that it was the patient’s choice to try an experimental drug), and allow them instead to receive the appropriate support for someone at the end of their life, and come to terms with that.  Further, it is important to remember that drugs at this stage are not necessarily miracle cures! If someone is refused access to a trial this is normally to reduce the risk of adverse consequences: it is wrong to give someone an experimental drug that could negatively impact the quality of their final days.