Alcove tub drain

Installing a new American Standard alcove tub and hooking up the drain. Its on the second floor so technically I could open up the dining room ceiling to connect drain but really really want to try and avoid that. One way I think I can avoid opening up the ceiling is by connecting the tub drain last. That is run all the PVC up to the tub itself, connect to the tub drain and then make the final connection to the tub itself. The manufacturer states if the tub drain is installed horizontally it will void the warranty. Why? Also do you think my method will work? I think I can reach around the tub sufficiently enough to tighten the tub drain - how tight does it need to be? Thanks for your help.

I have hooked up tubs before with out cutting the ceiling

Here is what you do.
In the garage, spare bedroom, Whereever
set the tub up in the corner. LEVEL it
Ido not CARE what the book says, during manufacturing the tub drain moves.

with your pencil mark the floor thru the hole in the tup
BOTH drain and over flow
measure from the floor to the bottom of the drain hole
measure from the floor to the center of the waste and over flow

remove the tub from the mock up wall

measure from the wall to the center of the drain hole you marked
go into the bathroom and plumb the waste and over flow.
using the numbers you got from the mock up

slide the tub into the alcove, being careful not to rip out the waste and over flow
BE CAREFUL. now is time for patience
IF you installed the drain correctly, all you have to do, is fish the rubber washer down thru the drain hole onto the shoe.
OR
dab a little contact cement on the washer so it wont move