many SA decockers are on the slide rather than the frame
The same argument could be made on the other side of the coin. I you have a decocker, carrying a gun cocked and locked is the redundancy and there is no need for a safety on an SA only gun. The decocked hammer/half cocked is now your safety. When you are ready to shoot, just cock the thing and it is just as easy as flipping off the safety. It is practically the same as cocked and locked in my mind.