Actually, this exact notice is also placed in some of the Java files Sun itself distributes. For example:

* @(#) 1.10 97/02/05
* Copyright (c) 1997 Sun Microsystems, Inc. All Rights Reserved.
* Sun grants you ("Licensee") a non-exclusive, royalty free, license to use,
* modify and redistribute this software in source and binary code form,
* This software is not designed or intended for use in on-line control of
* aircraft, air traffic, aircraft navigation or aircraft communications; or in
* the design, construction, operation or maintenance of any nuclear
* facility. Licensee represents and warrants that it will not use or
* redistribute the Software for such purposes.

Now as to why they have to put this in, I have no idea.