Why to their long term detriment? If someone is in an online depression support group, for example, and has issues with suicide, how is it detrimental to warn them about a post that contains talk of such things? There is plenty of research about the "contagious" nature of suicide for at-risk populations being exposed to stories and details about it. Allowing someone to make an informed choice about such content can literally save their life.

i avoid drinking poison, is that to my long term detriment?

it doesn't really matter what your personal opinion is on trigger warnings or PTSD is, if someone wishes to avoid a subject it is common courtesy to not bring it up.

people who have triggers are normally aware of their triggers and are hopefully going to therapy to learn coping techniques to deal with them.