They've got something in wrong or in the wrong order. You should be able to add in the correct amount of preload on the bearings and feel the resistance build up as you do so. The fact that it's an all or nothing deal means they screwed it up.

If you look at it yourself then look for the the usual cup and cone setup. Check to make sure the rounded cone faces are towards the bearings. Also be sure they installed the bearing rings with the cages facing the right way. If they put them in upside down on either side the cage won't fit between the cup and cone on the right angle.

Or if by some chance they are cartridge bearings you'll see that the faces of the inner race are larger on one side than the other. The larger face should face up on the upper race and down on the lower race.

Check for the split taper ring on the upper portion as well. I saw a bike arrive at a dealer and that part was missing for some reason. Without it you can't center the upper part of the stem and it'll rattle around like this if you loosen things up at all.