The trade deadline is when things change in the NHL. It's a great barrier point to work with: not only does it pretty much set playoff rosters, but it has an impact on non-playoff teams, too, as they're limited to four non-emergency recalls for the rest of the season.
So you wanted to reward the kids in your system and give them all a taste of NHL action? Too bad - the NHL doesn't want you to, so unless your entire team ends up injured, you have to pick and choose wisely.
The Leafs are kind of in that situation where everyone has ended up injured, anyway, but that hasn't stopped them from having to differentiate between who's an emergency recall and who's a regular one.