Harry's Muggle parents could not authorize it because they were Muggles, and Muggles had around the same legal standing as children or kittens: they were cute, so if you tortured them in public you could get arrested, but they weren't people. Some reluctant provision had been made for recognizing the parents of Muggleborns as human in a limited sense, but Harry's adoptive parents did not fall into that legal category.
I kinda doubt wizards in general care overmuch about Muggle law.
But presumably they do care about rampaging Muggles on witch-hunts?
That hasn't happened for about three hundred years. But then, by a mysterious coincidence, neither has torturing little children into insanity to appease the blood lust of the "nobility", something which is likely to get those pitchforks sharpened pretty damned fast.
"She is too young! Her mind would not withstand it! Not in three centuries has such a thing been done in Britain!"
Secrecy about magic does seem to matter to these folks, otherwise why go to all the effo...
EDIT: New discussion thread here.
This is a new thread to discuss Eliezer Yudkowsky's Harry Potter and the Methods of Rationality and anything related to it. With two chapters recently the previous thread has very quickly reached 500 comments. The latest chapter as of 17th March 2012 is Ch. 79.
There is now a site dedicated to the story at hpmor.com, which is now the place to go to find the authors notes and all sorts of other goodies. AdeleneDawner has kept an archive of Author's Notes. (This goes up to the notes for chapter 76, and is now not updating. The authors notes from chapter 77 onwards are on hpmor.com.)
The first 5 discussion threads are on the main page under the harry_potter tag. Threads 6 and on (including this one) are in the discussion section using its separate tag system. Also: one, two, three, four, five, six, seven, eight, nine, ten.
As a reminder, it's often useful to start your comment by indicating which chapter you are commenting on.
Spoiler Warning: this thread is full of spoilers. With few exceptions, spoilers for MOR and canon are fair game to post, without warning or rot13. More specifically: