With spoons, they don't come with anything attached. I don't know why. Some people will just use a snap swivel on the end of their line. I will take a #4 or #5 split ring and attach it to the spoon and then add a #7 barrel swivel to that. With spinners, I just tie onto the line. Sometimes they twist the heck out of your mono and then I wish I would have added a swivel somewhere. I have added a barrel swivel up the line like you said but that has resulted in a couple of ruined tips for me. A bonehead move, yes, because I forgot they were there and reeled in too far. Last year I even tried using a bead chain in my line for spinner fishing for salmon. That seemed to take care of the twist.
Incidentally, if you are fishing for salmon and steelhead, you will eventually find yourself releasing a fair number of fish. A lot of times it can be easier with a single point hook like a Siwash. A lot of us swap out the trebles for a Siwash on spinners and spoons.