This is pretty obvious in the general case, where it would be impossible for me to know if the URL I am visiting contains information about my particular user agent and is thus inappropriate to share with people with other user agents. When my user agent requests "en.wikipedia.org" and receives a redirect to "en.m.wikipedia.org" there's simply no way to know whether this resource has actually moved to a new URL (which is the semantics of the HTTP response) or whether this is simply a special URL that I shouldn't share. Yes, some people might be familiar with the "m." subdomain for mobile-specific websites, but in general this sort of thing is (in my opinion) an abuse of HTTP redirects.