I want you to know, LeenC, that I genuinely appreciate your perspective on this. I'm green to this board, and it's nice to be able to compare notes.
You're right--it shouldn't. And if there were some sort of agreement between the authorities and M regarding the protection of her identity, it isn't inconceivable that this very issue would have been extensively discussed.
First, she wasn't twelve at the time of the murder. She was fifteen. This is unimpeachable. Second, where did you specifically come by her own words that M's mother cut this boy's hair in an effort to conceal his identity? Is this paraphrasing courtesy of americasunknownchild.net, or even the Wikipedia article? Sounds like a lot of people are comparing this secondhand information against M's own testimony as related in David Stout's The Boy in the Box and asserting a contradiction where one does not empirically exist.
That, or his hair was kept long so that he could emulate the appearance of a girl in order to satisfy the desires of an individual (or individuals) besides M's mother. Admittedly conjectural, but I've not heard my idea proffered heretofore. Knowing M's familial background begs the question.
It's one thing to have formed one's own viewpoint based on information, but it is a wholly different cut of cloth to do so based on misinformation widely available on the Internet. And that's okay--few people drill into this case anymore, sadly. Fewer still are willing to drill as deeply into M's account, unfortunately.