As regards the cutoff for the SMLE, if you take a close look at one, you will see that it was emphatically NOT just "stamped out". It was carefully machined to a very precise shape, then hardened and spring-tempered to be its own catch and spring. Making the slot in the body was the easy part. Drilling and tapping the screw-hole was harder. Making the actual cut-off itself was slow, fiddly and expensive.
As to the regulations, I would not be surprised if they changed, over time. I know for a fact that some troopies in the Great War were instructed to keep the cutoff engaged and single-feed until they needed the full magazine. And on the Mark II Lee-Metford, which had no safety apart from the half-#### notch, the cutoff worked fine to assure that no round entered the chamber unless you wanted it to.