Wow I've gotta be missing something! I played with it for the longest time and don't see how it clamps, with the washer in there and constantly screwing/unscrewing the thing I couldn't get it to give me any resistance when I gave it a little tug.. I'll need to check that out more I guess, it should be solid/noticable change when I screw it into place? I was curious why it even had that threaded spot because I couldn't get it to do anything when I screwed it on/off with that outlet spout in it.
It is not common practice to be loose, something is wrong with the connection. The pvc is solid enough if put in right...I can hardly picture a cat knocking it off. I would replace it all. Parts needed: end out waste kit (that is the "T" and the long pipe that keeps falling down)that sometimes comes with tail pieces about and inch long but you can buy a double ended tail piece and cut them as long as you like, also get a p-trap. Size of kitchen sink waste lines is 1 1/2 tubular...no need to go any heavier, that is exactly what you have now. Cost about 10-15 dollars tops. It helps to brush a little pipe dope on the washers to prevent leaks but not required. Tools required: hack saw and channel locks.
I really don't know why you are dragging this thing out, quit trying to reuse the existing parts...evidently the nut is not tightening down as it should. Buy new and replace. Easy job if you cut the tubes to the proper length, slide nut on first, next, plastic washer with tapered side down, brush a little pipe dope to be sure and tighten by hand and snug with channel locks if you like.