They are hardly the only site to do that. It is only by convention that non-www-prefixed websites tend to redirect to www-prefixed one. It's not at all mandat

They are hardly the only site to do that. It is only by convention that non-www-prefixed websites tend to redirect to www-prefixed one. It's not at all mandatory and technically they are two different hostnames. It may be intentional to ensure only exactly the right hostnames are ever used, e.g., www, ftp, smtp, etc.