You're all right. It would be very easy to just upload a short 20 minute set to youtube and call it a day. BUT the situation it very different from the Papercuts "era", which basically was a celebration of the classic LP era. This is a matter of being coherent with your message. Since September the band has made very clear that they're looking forward to their future, not being locked in the past, albeit being respectful with it. Releasing that old show would just cast the image of a band unable to let go its past, even in the middle of a new release and a huge world tour. No matter how many things they do, they're stuck in that era.
And yes, we all know that in reality just a short show wouldn't undermine the new era of LP and this is just a good gesture with the fans, but we are the hardcore fans, we are a tiny minority and many people out there don't think like us.