Executing commands without sudo

[Gary Call]

I just installed sudo on a SCO Openserver 5.05 server.  The
configuration file has been edited via visudo. 

Here's my problem:
A user ("john") logs in.  In visudo, his entry looks like:
john	ALL:/bin/mt,/bin/cpio

When I log in as john, I get the following:
prompt$ /etc/mount  (this allows him to use mount)
prompt$ sudo /etc/mount (this denies him via sudo)

Logging in as john, I can issue just about every command without the
command being prefixed with "sudo".  Once I place sudo in front, I am
then denied.

Why is the user "john" able to execute everything when he is not
prefixing the command with sudo?  I was under the assumption that
everything for "john" would be denied, unless he prefixed the command
with sudo.  Once he prefixes the command with sudo, he would then be
allowed only /bin/mt and /bin/cpio.

I need serious help!  Any suggestions, help, pointers, jokes, etc..
would be greatly appreciated.

Thanks,
Gary