Now, obviously, it's not about not wanting any of the Leafs' players to go and represent their country, because
Timothy Liljegren was granted and is currently playing with Sweden now. This seems to be more about not wanting to risk an injury with a potential trade coming up in Toronto. Nylander and teammate
Mitch Marner appear to be the two members of the Core 4 who make the most sense to deal. If Nylander were to suffer a serious injury in the tournament, it would limit what the Leafs are able to do this summer, so in that sense, it makes a lot of sense not to allow him to go.