Falling is a standard form of villain death in comics, movies, etc. It's notably common in Disney movies as a way of dispatching villains, because it's discreet; the death is not actually shown, so it can be suitable for kids, but it's still left pretty clear that the villain won't be coming back. TV Tropes actually calls this the Disney Villain Death
. (See also the end of Superman II
.) It can also be a handy way of killing off a character without the heroes getting blood on their hands (often the hero tries to save the villain but fails). But it's ambiguous enough that it's also suitable for a character like the Joker, who's constantly seeming to die and then turning up alive in later stories. Other such characters, like Ernst Stavro Blofeld and MacGyver
's serial-killer nemesis Murdoc, have been known to fall to their "deaths" on occasion. As long as their death is unseen and no body is found, it leaves a door open. And since the Joker is pretty much the champion of fakeout deaths, he tends to fall off things a lot.