Bloom is spot on.
I want to add a couple of points. First, an aftermarket roof rack is even worse than factory racks...they are held on by sheet metal screws. Oftentimes, racks are added by dealers, and while the rack may look like OE, it is not. Bottom line, you really can't know for sure.
Second, webbing under load can eat right through paint. I wouldn't let the webbing touch anything I cared about.