The implementation of ExternalContext.redirect() encodes the URL that is passed to the method, although it must not. This behavior is not mentioned in the JSF specification, and must not be performed.

If you encode yourself the URL passed in as a String to redirect(), this means that you consider that the URL is not encoded by the caller. Then, how can you correctly parse that URL ? More precisely, if the URL is not encoded, how do you distinguish that the '?' character found in the URL is actually the separator between the path and the query string, or a character in the value of a query string parameter.

As an example, the following URL

/redirector/goto?newUrl=http://some.site.com/pages?id=12

is parsed by BridgeExternalContext into an URI with the following characteristics:
path = /redirector/goto?newUrl=http://some.site.com/pages
querystring = id=12
and then encoded into this:

/redirector/goto%3FnewUrl=http://some.site.com/pages?id=12

This may or may not be correct. Only the client knows it. The client actually makes this clear by encoding the URL before passing it to redirect(). In my example, this means:

/redirector/goto?newUrl=http://some.site.com/pages%3Fid=12

Of course, ExternalContext.redirect() must not re-encode the passed in URL, or we will get something like:
/redirector/goto?newUrl=http://some.site.com/pages%253Fid=12
because %3F is encoded as %253F.