MC groups and motorcycle policemen ride side by side. The group of four may or may not be. The eighteen is almost guaranteed.
I do ride staggered usually as that is how the people around me like to ride. I seriously doubt the assumption that side by side is less safe than staggered for good riders.
The key word is "good" which I interpret to mean "experienced" riders. If I remember correctly, the Motorcycle Safety Foundation says that riding staggered is the safest riding method.
https://www.msf-usa.org/downloads/Group_Ride.pdfMotorcycle Police riders are, in my opinion, highly trained as compared to the average rider. They (Motorcycle Police) can ride any way they would like.
Now one has to define "good" or experienced. Does it mean number of miles per year, number of opportunities for error experienced?
I have yet to ride with a group of riders that don't ride staggered but I will admit, I don't do a lot of "group" riding (group being more than three bikes).