As I said above, you can't really keep a critical mass - and therefore keep safe - without ignoring some traffic laws. I'd be interested to know (genuinely, this isn't just rhetoric) how you'd conceive of something like this happening "in a coordinated and legal manner".

They're protesting against infrastructure and laws that they see as anti-cyclist. Following the laws they're protesting against is generally an ineffective tactic, although it has also been done (with large groups of bicyclists diligently following traffic laws, which causes massive traffic holdups).