We don’t really know who’s at fault here. On the one hand, it makes sense for those nearest the front of lines to get out of their queue and head to the next open register. On the other hand, it can be hard for them to leave their queue because of all the stuff they bought. In that sense, it would be more reasonable for those at the back of long queues to move to a new register.
However, if you really think about it, what’s reasonable isn’t always fair. Sure, it’s a pain in everyone’s butt when the guy at the back of the line is the first in a queue at a newly opened register. But is it really that guy’s fault?