That is basically the textbook definition of unethical medical practice, so unquestionably far over the line of acceptable practice that you would have to be willfully ignorant to defend it, and they think it would be exciting if it were not banned.
This is aside from the harm it does to the rest of us to prevent experimentation by willing participants, such as barring human challenge trials to quickly test Covid vaccines.
i would guess that these protections exist to cover a broader group including children or those who are in the care of others and aren't necessarily capable of making their own decisions about experimental treatment... to say nothing of other forms of coercion otherwise-capable adults may face when it comes to stuff like this.
it's tricky! and it doesn't seem like there's a one-size-fits-all approach that offers protection for those who need it.