Trigger warnings are misunderstood by many. Sometimes things hurt the most when they come as a surprise. Whereas if the person is warned ahead of time, they can better prepare themselves mentally.
Trigger warnings don't HURT people's ability to cope with their trauma - trigger warnings actually HELP them deal with it on a daily basis.
Admittedly, I've only seen one book by a major publisher that as a warning on it: Lilah Pace's ASKING FOR IT. But warnings are common with self-published and small-published books, especially in the romance genre.
That Hoover would rather surprise her readers with traumatic flashbacks than have her characters' secrets "spoiled" says a lot. Namely that she uses trauma as a plot device. And also that she cares more about her fictional characters than real people.