Thats because (mild spoiler for the books) every young person has "the trace" put on them, which can be tracked. Any magic done in the vicinity of someone with the trace on will be picked up on. That said, they are apparently aware that it was a hover charm in book 2, so they can clearly detect the type of magic too...
That's something in the original universe which seems unrealistic to me, so I guess it doesn't work exactly that way in MoR. Someone in the ministry being warned, with the details of the spell used, for every spell used around an underage would mean, de facto, being warned of almost every spell usage done by any parent. It would mean that every spell cast by Lucius when Draco is nearby is detected by the ministry. I doubt both the Death Eaters and the "normal" wizards would accept something like that.
Also, in canon Order of the Phoenix, near the ...
This is a new thread to discuss Eliezer Yudkowsky’s Harry Potter and the Methods of Rationality and anything related to it. This thread is intended for discussing chapter 90. The previous thread has passed 750 comments.
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: 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17,18,19.
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: