Why 0x1 is diallowed in URL

GET /P%EF%BF%BD%EF%BF%BD%01 HTTP/1.0 Host: www.XXXXXX.com Accept: / User-Agent: Mozilla/4.0 (compatible; MSIE 8.0; Windows NT 5.1; Trident/4.0; .NET CLR 2.0.50727; .NET CLR 3.0.04506.648; .NET CLR 3....