I wonder exactly why that is, except of course for that being the default standard way of putting the taper pins in.
I had to get a new A2 sight tower (from Fulton Armory, Bushmaster wouldn't sell a part to me that requires gunsmithing because they wouldn't want to take a return on a part I might screw up) because I had picked up a Busy 20" national match barrel for a great price, but the sight tower was gone.
I have a small milling machine and plenty of measurement tools, and the right reamer, so I was able to drill and ream a set of holes in the tower to exactly match the sight tower to the existing holes in the barrel. It's not a job you should attempt without a mill and some related tools. It went quite well, and the rifle shoots fine, with the sights centered as well as your average AR.
I'll tell you one thing, though: If you go to take out the taper pins, your best bet is to clamp the barrel very securely in a massive vise and use a very good, well finished flat faced punch of the correct diameter when you take the pins out, because if you don't get a true hit on the end of the taper pins, you may start mushrooming the small end, in which case you're going to be grinding with a Dremel or maybe even drilling the pins out. That'd be a bad scene, so make sure your barrel is rigidly held and make sure the pins get hit accurately and cleanly with a sharp but not excessively hard blow. If you have to hit them more than 3 or 5 times, they're already mushrooming.
The pins are inserted from the right side of the tower. That's SHOOTER'S right. So drive them out from the left.
At least, I have yet to notice an AR where this was incorrect.
CJ