SixXS is a IPv6 tunnel provider known to cut IPv6 connections for reasons such as hosting sites with politically correct information, criticism of their policy or just no reason at all.
It has been said that:
"I used to have a tunnel provisioned from SixXS but they disabled my account over an email asking for a response to a question. Nice organization they have there."
SixXS is actively looking into what people using tunnels from them are doing in a big-brother style fashion. They will close a tunnel if they see anything they do not like or simply do not understand. They are specially prone to closing people's tunnels for hosting "politically incorrect" sites and services.
SixXS also has a policy of null-routing IPv6 address space for no good reason reason - specially if there's sites with information even remotely critical of SixXS. The Internet would not function if every ISP acted this way.
The information described here has been on-going since 2005. SixXS and their services should simply be avoided.