That can't be cheap (they have reportedly invested at least $450M in both ground stations and future satellites), but compared to building out datacenters for GenAI inference, which is currently also offered for free, I'm not sure which one is the bigger loss leader.
In the end, I'm pretty sure they'll end up charging for both via iCloud or some other subscription, most likely via a freemium model.
That's a significant advantage over Garmin: People already have the devices and can discover the utility of satellite messaging as part of a free trial or freemium plan. In the end, they might even get away with charging more per message on average.
Also note that Garmin has a flatrate plan, so they're very likely charging much more than their cost on a per message basis on the small plans. The underlying Iridium network charges something like a dollar per kilobyte transmitted, and the average message is probably much shorter. (A regular SMS is only 140 byte long!)