You’re absolutely correct! OS X includes /sbin/md5, however it’s output is different than the md5sum utility:
$ md5 myfile.iso
MD5 (myfile.iso) = 3d763a372e34681287712f661ecb598c
$ md5sum myfile.iso
Many scripts that utilize the md5sum utility expect the output to be in a particular format. If the format is different (as it is with the md5 command), it will break the scripts that use it.