This is a message board, topics always leak and diversify, as they should. Gives discussions scope and accuracy. For example, this thread caused a lot of people to suggest GT as a franchise was dead, dying, did terribly etc (not limited to just the US) and now we have shipped numbers of 5.5 million for 12 days.
Puts a completely different perspective on things no? And I don't know if you should use your own opinion to instill what "most" people want to discuss. NPD's are currently the best avenue to discuss mainly NPD focused, but also regionally over arching sales. Then again, that happens in PAL and Japan sales threads too.
There is no Draconian rule (with good reason) that regional sales threads should only have posts specifically related to that exact region or topic. In this thread I've seen everything from discussions on specific game development, sales threads from other generations and dates, sales from different regions, specific publishers, specific platform sales sales etc etc.