You know, the anime studios and the TV stations depend on the viewers, that is, customers. They are professionals in the business. If previews were as catastrophic as you said, they wouldn't exist in the first place. They aren't meant to spoil, they are meant to make you look forward to the next episode, to be excited. Unfortunately especially in filler hell anime or poorly written ones the previews tend to be better than the actual episodes, but then again, nobody would complain about the previews in such shows.