The correct way to ride in a group is staggered. You are supposed to ride in either the right or left side of the lane where car tires ride, especially when raining, but it's really up to the rider and whatever his/her comfort zone is. That is also void when riding back curvy roads.
Riding staggered allows each motorcyclist to swerve left or right if debris is encountered in the road, and also creates a cushion area in front and behind each rider should someone slam their brakes on or something - you dont run into them, you run up beside them or on past them.
Regarding riding side by side, only time I do that is on slow roads in town, no need to be staggered and stretched out, especially with several riders, while going 15 mph through a downtown area.
Everybody rides different, but as long as they are contained within their lane it's legal, so I don't see the issue? On the way back home from Lynchburg last year over Skyline mountain, we followed two guys on Harleys like those in your picture and they rode side by side at 65 mph the entire way back. Lessens your room for error, no way I would ride at highway speeds side by side like that.
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