I can see logic in holding up the traffic for a few moments to allow the bikes to complete their turning as a block rather than intermingling cars and bikes en-mass or having the group broken up into a large number of subgroups. Considering all the bikes that they pass on the way it looks like its a pretty large convention either formal or informal. Sometimes the rules of the road have to bend a little to accommodate practicalities of reality. Though it would have been better to have had police attend to help organise the flow of bikes if at all possible.
To me its one of those things where if it happens once its probably fine, its more an issue if they were holding up multiple turnings over the day and causing undue stress, confusion and holdups to the local traffic on a larger scale.