What if Amanda had brought Maniac home to her house and it was Sam's mom from iCarly instead of Mrs. Beale? There's a reason Maniac is able to stay with the Beales so long when he first shows up in Two Mills, and that reason's name is Mrs. Beale.
Mrs. Beale provides everything that Maniac has been missing ever since his parents died: unconditional support, encouragement and love. That doesn't mean that she indulges him no matter what, however.
Maniac learns this lesson when he directs a little smack talk in Mrs. Beale's direction. A real mom will let you know when you've messed up, and Mrs. Beale does, right away. But what really shows us what a true family they've become is the way that both of them react to Mrs. Beale's actual smackdown:
Her lip started to quiver before his, but before she could say 'I'm sorry,' he was hugging and squeezing her and burying his face in her chest and sobbing, 'I love you...I love you...' (15.19)
So we know Mrs. Beale feels unconditional love, and we know that Maniac understands what this means: a real parent will teach you what's right, and teach you what's wrong, and keep you on the right path. And they will always, always love you.
After leaving the Beales, Maniac faces some tough obstacles. He even seems pretty close to giving up: "Knowing it would not be fast or easy, and wanting, deserving nothing less, grimly, patiently, he waited for death" (33.12). But he can always remember the Beales. He can always know that he has a family that will welcome him with open arms and offer him love no matter what.