I'm sure it CAN go on finished drywall. I, personally, would not tile onto finished drywall. You want to cement/glue the tiles to the wall, not the paint and texture on the wall.
years ago when doing my first bathroom renovation I ran out of hardibacker and was too cheap to buy another two sheets - so I strung up some drywall instead and tiled over it. 4x4 - floor to ceiling - hangs just fine even considering the humidity. I've had that wall open since then (fro mthe other side) to do drywall in the adjoining room and noticed no problems).
But I don't recommend it necessarily, I'm expecting it to fail sooner than the areas where I tiled properly onto hardibacker. Yet it can be done and will adhere.
