The logic isn't "it comes from seaweed so it's not harmful" it's "you wouldn't really give a second thought as to whether seaweed salt was safe to consume if you saw it on the shelf but are naturally skeptical of MSG because it's a scary chemical name."
And that's a perfectly good argument.
"It comes from seaweed and you eat seaweed just fine" is not. You've singled out arsenic in your response, but I also mentioned formaldehyde, which is throughout the apple's flesh. Would you drink a bottle of it because of that?