Hi John, Maybe because our was a rural league,
http://www.belltarget.com/lord-ednam.php
with many farm workers (still has a fair few.....saw two guys literally straight from a muddy field one year when they were working late on harvest) That we are open sights only, and always have been I have been told.
It seems perhaps because of the extra cost back in the day of apertures so they wanted to keep it accessable. I don't know this is 100% correct but will try and find out as there are records going right back as far I am aware. I have seen scorebooks back to the war, but never the original gun specification other than "BSA Air rifles".
All I know is they are very strict about anything other than open sights, even sight hoods have to be removed! and 100% no shooting type clothing or indeed anything that can be construed as a "shooting aid, no butts inside jackets etc. etc.
There again we use only recoilling guns of full power (up to 11.6) so maybe it's just a "in the sticks" thing!.......I think the targets are set at least at 6 pounds, so no chance of any match rifles getting a ding!
ATB, Ed