> If you grab a megaphone and start disrupting a whole factory floor by using it to try to get people to organize for better rights while you're supposed to be working, that seems like it ought to be a fireable offense, where organizing at someone's house outside of working hours clearly should not.

Can you organize on company time? It depends - can you discuss literally anything other than work while at work? Then yes. If not, then you'd have to organize off the clock but not necessarily off-site. There are exceptions for reasonability sake (a nurse can't interrupt an organ transplant to organize, for instance) but sending company-wide emails strikes me as sufficiently non-disruptive as to be protected.