You are likely right, because it is usually less of an issue with a kettle as there often isn't a much moisture. I notice that Weber seems to give a cover with a new WSM & it is probably because of the rain issue. Whether a kettle or a smoker, you don't want a much of water mixing with the ashes.
With the kettles, at first I assumed rain was getting through the top vent, into the kettle, & then out through the bottom into the ash bucket. Was trying to figure out how so much water got through there. After thinking about the WSM, I realized the rain is not getting into the vent, but running down the outside of the kettle & a decent percentage just streams along the curved surface until it is above the bucket and most of it just streams right in. At that point, I realized why you see (reasonably often) pictures of a used performer or master touch with the bucket removed & sitting underneath upside down.
Empty that bucket of ash & then remove it so the rain water doesn't sit in there with whatever ash is left messing up the metal.