If someone throws a fit because they involuntarily read an incest story on a site full of porn, they only have themselves to blame.
I think you miss the point of tags being used as trigger warnings. Some people in this world have experienced a shitload of trauma in their past. This leaves them lingering with post-traumatic stress disorder, anxiety disorders, panic attacks and some other psychiatric conditions. Acute episodes of such diseases are horrifying (I speak from experience), and they are sometimes triggered by situations which resemble the origin of the trauma.
So if I unknowingly read a story with rape in it, I'm at least 50% likely to have a panic attack. Sometimes it's light enough that'll just end up losing a few hours of sleep. Sometimes they make me wanna die.
The thing about triggers is that they're specific for each person. So for me, rape is almost always a trigger, but so is sex with robots or dolls (because in my case, I felt like someone trapped inside an inanimate object being used by someone else for their pleasure). People who have suffered violence by family members might have incest be a PTSD trigger. And so on.
This might sound like some rare occasional thing, but keep in mind that 1 in 6 american women are rape victims. The number goes up even more when we're talking about LGBT people (1 in 5 lesbians, and 1 in 2 bisexual women, if I'm not mistaken). And rape's just one form of violence, there are several more which can be equally damaging to your mental health.
We use trigger warnings in order not to hurt the people who have already been through too much. I think you'll agree with me that this is a more important cause than preventing spoilers.