Depending on how well the windshield people installed the trim, there could be one or a combination of ways they are attached. they could have used some urethane goo just to stick into place, they could have used screws instead of rivets, or they could just be completely loose.
2001LC, among other amazing in depth posts, shows himself cleaning and reattaching these very pieces
here. I have this same problem in mine, but have not got around to addressing it yet. Most of the time the windshield people do install the windshield properly, so there's probably no leak into the actual cabin but I suppose loose trim would allow water to be trapped against the body and cause rust on the pillar.