I have no idea of the official reason but:

1) She was an antihero - emphasis on the hero. As dark as the film was it was still a time where the hero had to live. (in turn may boost rewatches, video sales etc) 2) Spinoff potential. I heard that there were plans for a Black Cat standalone (before the Halle Berry one).

I imagine the bottom line was that one way or another, by having her live they would make back the million bucks back and then some

The reason for it, I don't know for sure, but I do recall hearing that the last shot of her standing up was some kind of mechanical thing, not Pfeiffer.

Could be accidental. I heard the same thing happened with Buffy where they were gonna kill off a character but they kept breathing to much. So they kept them alive.

I would've loved a black cat movie in the same style

Money

@A-Dubya said:

The reason for it, I don't know for sure, but I do recall hearing that the last shot of her standing up was some kind of mechanical thing, not Pfeiffer.

I've heard that too, but the Catwoman prop looked terrible and it was reshot using a stand-in.