Deve ser alguma checagem de user agent, ou algo do tipo. Print anexado.
Dados relevantes do request:
URL:
Bem vindo a Bastter.comUser-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.10; rv:37.0) Gecko/20100101 Firefox/37.0
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
Pra ser mais claro, estou usando Firefox Developer Edition, no Mac OS X.
Resposta que recebo (headers):
HTTP/1.1 302 Found
Cache-Control: private
Content-Type: text/html; charset=utf-8
Location:
http://m.bastter.comServer: Microsoft-IIS/8.0
X-AspNet-Version: 2.0.50727
X-Powered-By: ASP.NET
X-Powered-By-Plesk: PleskWin
Access-Control-Allow-Methods: GET,POST,OPTIONS
access-control-allow-headers: content-type
Access-Control-Allow-Origin: *
Date: Fri, 30 Jan 2015 23:13:45 GMT
Content-Length: 13880
Ou seja, redirect para m.bastter.com