Look at it this way. Leafs gave up a 3rd round pick for a player that might not play for them in the playoffs, at all. The condition under which he'll probably not play for them in the play-offs, is the situation in which Andersen becomes 100% healthy for playoff time. He's a depth option that may never play an important game.
If he's waived, it means everyone got healthy for the play-offs. Great! He'll probably not play, either because he's claimed, or because we never had to go down to our 3rd stringer.
Teams pay 3rd round picks for players that sit on the bench in the playoffs all the time.