Maniac does not care know about the local legend and he just wants to help the kid.
Jeffrey “Maniac” Magee is new in town. He went on the run when he was orphaned and did not like the relatives he got stuck with. Being new in town, he is not aware of all of the local legends, including the fact that no kid goes anywhere near Finsterwald's place.
What Maniac saw was a bunch of older kids bulling Arnold. He helped Arnold, or at least tried to.
[Maniac] stuck the book between his teeth, crouched down, hoisted Arnold Jones's limp carcass over his shoulder, and hauled him out of there like a sack of flour. Unfortunately, he chose to put Arnold down at the one spot in town as bad as Finsterwald's backyard -- namely, Finsterwald's front steps. (Ch. 5)
Maniac was not afraid of anything. He wasn’t afraid of a group of older, bigger kids, and he wasn’t afraid of Finsterwald's house. It seemed as good a place as any to stop and rest, so he sat there and read his book. This pretty much ensured no one would mess with Arnold anymore. They were too surprised.
As the stupefied high-schoolers were leaving the scene, they looked back. They saw the kid, cool times ten, stretch out on the forbidden steps and open his book to read. (Ch. 5)
Maniac was “cool times ten” there on the porch reading a book. And of course, this is how legends start. Maniac was a pretty ordinary kid except for the fact that he was fearless and openhearted. He demonstrates both of these traits in the story of Arnold at the Finsterwald's.
Although Jeffrey repeatedly gets himself into situations that build the legend of Maniac Magee, he never seeks them out. He is just a kid looking for some place to belong. The fact that he does not back down from any challenge and faces life with a positive attitude just makes the legend grow.