My guess would be it's to build up reputation of the sending accounts, by being "connected" to trusted, aged, accounts? However, the combination of garbled text + number @ large provider on its own should be pretty suspicious by now. Another idea is some kind of affiliate marketing / pumping attack, but I don't see ads.
Kind of out there, but if you were running a network of agents that are autonomously looking for exploits, then perhaps those agents have decided that a good source of fresh ideas might be to periodically receive Schneier’s newsletter in an inbox they control.
> All Gmail. None of the addresses has actually subscribed to Crypto-Gram. They could; whoever is sending the emails could easily have confirmed the subscription.