Nobody is forcing you to watch anything! The power is always in the consumer's hands, you can always turn it off or stop watching when something "triggers" or upsets you.
Don't you think it'd be good to know beforehand though? It wouldn't negatively affect anything so there isn't really much of a reason to exclude something helpful like this. If I had, say, arachnophobia I certainly would appreciate a warning before the episode starts so I could prepare myself and know that spiders/etc are going to appear in this episode, or just change the channel or do something else.