This is a type of question that tends to get downvotes. The structure is :
- The reason for a certain law is X Y and Z.
- But X Y and Z are not good reasons for this law
- Therefore this law is wrong.
- Prove me wrong...
There are lots of questions like this about drug law
What's the point of legalising dangerous drugs in some states in the US?
Why are many safe narcotics illegal?
Why very few countries/states try taxing and legalizing safe soft drugs?
And more that have been closed and deleted.
Your question seems similar. It makes a supposition "The reasons for informed consent are questionable". Then it presents a long quote which may present this argument. Then it seems to ask for a counterargument.
The problem with the drugs questions is that they aren't really inquiries into a political process, but an attempt to argue a point. They are a disgused form of "I think drugs should be legal and here is why..." (and I'm not actually interested in the answers).
Now on closer reading, your quote doesn't actually present an argument against informed consent, and that was the basis of my answer (which essentially just summarised the quote).
Moreover this is a strongly emotional topic. And you seem to be arguing agaist informed consent. If you seem to be taking an unpopular point of view, you can expect to be unpopular.
I think much of the downvotes come from the title "If the justifications for informed consent are questionable, then why would it be a legal requirement?" As this doesn't summarise the question well. It seems to be assuming that "the justifications are questionable". It seems to be presenting an argument. The quote you give doesn't seem to present that argument, so some downvoters might feel you can answer the question merely by reading your own source carefully.
A better title, focussed on what exactly you don't understand about the source could have got a more positive response.