Are All AR-15 Bolts Manufacturered This Way ?

[billt]

http://www.youtube.com/user/billt460?feature=mhum#p/f/6/4mPfhBmXq_Y

At the 3:40 minute mark in this video he demonstrates how the bolt cam pin cannot be inserted into the BCG 180 degrees the wrong way. I noticed on all of my D.I. AR-15's this is the case also. Does anyone have a AR-15 that can be put together "backwards" in this regard? I was just curious. I thought this was one of the better AR-15 cleaning videos I've come across by the way. It is in 2 parts. This is part 2 of 2.  Bill T.