The airline's only concern, really, is liability. Their decision, I suspect, is that there is no real mask mandate present (legally). Therefore, they can't be

The airline's only concern, really, is liability. Their decision, I suspect, is that there is no real mask mandate present (legally). Therefore, they can't be held liable for anything that occurs with or due to a missing mask as there's no legal alternative to no-mask. People, being free agents, can choose to wear masks or engage is other public health measures and companies might even suggest such actions. And, as various professional athletes are finding out, what applies domestically isn't necessarily relevant internationally - there's been 0-5 players failing to make it to Toronto to play game(s), and in baseball at least, they're forfeiting salary.