Am I misreading or are you chrooting to an x86 build of Raspbian?
I don't know how exactly this sorcery works either. But I think chroot just "knows" that it needs to invoke qemu when it sees the binfmt of /bin/bash being arm.
What's happening here is binfmt-support on the host has a path registered for arm binaries as the file you just copied to /usr/bin - whenever the chroot tries to run anything, the host directs it to this bianry which is your arm emulator :).