I don't know what to tell you. Normal users can't tcpdump loopback on OSX, or anywhere else.
> $ ls -l /dev/bpf*
crw------- 1 root wheel 23, 0 Feb 29 07:59 /dev/bpf0
crw------- 1 root wheel 23, 1 Feb 29 07:59 /dev/bpf1
crw------- 1 root wheel 23, 2 Mar 2 11:11 /dev/bpf2
crw------- 1 root wheel 23, 3 Mar 2 10:07 /dev/bpf3
crw------- 1 root wheel 23, 4 Feb 29 08:11 /dev/bpf4