The referer might be null (i.e., there is no referer header) if you type in the URL in the browser, or if you access the page through a bookmark. Apparently, the author of this page does not want a user to start at any other page than home.jsp.
This is not a good design, because it assumes that the referer is sent, when in reality it may not be sent (e.g. if you crank security restrictions all the way up in IE, or if you set the associated configuration item in Firefox, then those browsers simply won't send the header).
He was expelled for perverse baking experiments. This tiny ad is a model student: