The thing is, previews aren't spoilers. They are released at the same time as the content. They are part of the episode. It is basically a teaser to invite viewers to watch the next episode. There really is no decent argument that will support making previews as actual spoilers.

I am willing to adjust so that those who do not watch previews are not "spoiled", but that does not mean that I believe that previews are spoilers. It just means that I can compromise because I never felt that strongly in the first place.