It's not really that cheap, guys XD
Basically, it puts him in a lot of danger.
If he's on the edge and you're preventing him from getting back onto the stage, and he decides to fly under and to the other side, what's he gain from it? All he does is get back on the other ledge, and you're there preventing him again.
On the other hand, if you can get off the stage quick enough and land a hit on him before he can grab the edge, you'll cancel his up+b and get a free kill. I honestly kinda prefer when Light's flying under because of that, even if it's extremely rare for me to catch him before he can get the ledge.