Out of curiousity what is be official "justification" for a site ban here? Haven't polygon flaunted it enough for it to happen now? There's literally blatant personal bias on display with no shame.
Generally, inaccuracy.
A number of sites have been banned from being posted on GAF. Trying to post one will get the URL replaced with asterisks. The reasons for each individual site ban vary, and new sites are added and old ones removed every so often. In general, sites are blocked after either a history of posting false information, or a history of self-promotion. If you notice that you've posted a banned site, just edit your post and look for a better source.
If you're reading a HUGE rumour ("Nintendo going third party") from some random Blog you've never heard of, use some common sense. If you don't believe the rumour yourself, don't post it. Posting something obviously crazy and calling it a rumour is not a defence.
One site in particular, VG Chartz, tends to come up a lot. VG Chartz numbers are considered unreliable, and because sales discussions often degrade into console war-style arguments between posters, it's really important to us that accurate sales numbers are posted for discussion. Users are generally banned for posting VG Chartz numbers because of the way it derails threads and prevents good discussion.
I think there is plenty of grounds for a Polygon ban, and there has been for months.