JavaScript - Copyright

I often see copyrighted javascripts. Like below. Why can't i delete the copyrights, because if i delete it, the script wont work.
(Iam not trying to steal the script but i want to use it with a other script.


Quote:
/*
*.......................- By JavaScript Kit (http://www.javascriptkit.com)
* Based on code by Adam Brown
* This notice MUST stay intact for use
* Visit JavaScript Kit (http://www.javascriptkit.com) for this script and more
*/