chflags fails with immutable flag

[Mail Man <mailman_(_at_)_amadeuss_(_dot_)_net>]

I'm testing the costs/benefits of using immutable files and I've run into a 
few errors I do not understand.  I'm trying to lock down most of the binary 
files on the system, but for some reason chflags fails with the same couple 
of files every time.  When I do a 'chflags -R schg /usr', I get a 'Operation 
not permitted' on the following files every time:

/usr/libexec/auth/login_crypto
/usr/libexec/auth/login_snk
/usr/libexec/auth/login_token

Does anyone know why just these files are causing problems?