Can anyone think of a reason the organizers would have decided to use workplace email for this?
And a large part of organizing is bringing people into the fold, so you'd still need to be organizing at work somehow, even if all you're doing is asking for external communications methods to continue the conversation. There's a much higher drop rate that way though.
Legal protections are not very helpful in a context where the other party has unlimited legal resources and time is not on your side.