The manga analogy does not work. They are different mediums. The preview is part of the episode. If you don't watch it, that's fine. But that does not mean you should stop or complain against people who have seen it and want to discuss it from doing so. Stopping me from doing just that isa dick move and being an asssomething I cannot agree with, because you are preventing me from discussing something I want to talk about that is well within forum rules just because you don't want to listen to it.
Discussing previews is also another way of enjoying anime. The preview does not completely spoil the content, and allows for speculation which is also a fun way to appreciate the show. The previews sometimes even fakes out the audience, making you think one thing will happen when it will go a different route.
This does not mean that I am not willing to stop discussing previews. It just takes a simple polite request. Calling a perfectly valid post a dick move and being sarcastic about it isn't swaying me to your side. I never had the intention of spoiling content to anyone, since I thought most people watched the previews, like I previously stated.