The CM web sites last time I looked at them in any detail were all quite clear about saying "this is not an official web site, this is the personal views of someone or some people who go on CM a lot". It may be that they've changed since then, but as Ben points out the custom is pretty well entrenched now anyway.
I'm no particular fan of CM, to tell the truth, simply because I tried it once and it was tediously slow and to be honest if the goal is just to have fun cycling in London why pick Friday rush hour as a good time to do that? The absence of formal organisation means you get ad-hoc organisation, which amounts to "whoever shouts the loudest wins" and "there is no accountability of decisions made". It's the Tyranny of Structurelessness
they're riding on public streets, and city streets are not just a way of getting from A to B, they're important social spaces - and you cannot have a sensible discussion of the merits or otherwise of holding parties for cyclists in the middle of them without considering that. There are bigger considerations than impeding traffic flow.