The copyright and license notice is already in the LICENSE file. There
is no justifiable reason to also require that it be included in every
file, since the individual files are not individually distributed except
as part of the entire package.
The output of `id -G` is unreliable on OS X. It uses an undocumented
Libsystem function called getgrouplist_2() that includes some auxiliary
groups that the POSIX getgroups() function does not return.
Or rather, not always. It leads to fun bug chases where the test fails
in one terminal but not in another.