I agree it is, but right now it's very difficult to have the discussions necessary to draw those lines—supporters of trigger warnings only want to discuss the cases where they're clearly justified (fireworks & veterans) and pretend all uses are similarly valid, and critics of trigger warnings only want to discuss the cases where they're clearly not (slimy things), and pretend that all uses are similarly invalid. Determining where the lines should be drawn will be a long, complicated, painful discussion, but currently there's no way to have that conversation. If your opinion is somewhere in the middle, most internet commentators will lump you into the all-for or all-against group.
When something is 1) subjective or very hard to measure and 2) used as an emotional bludgeon then then people are going to call "shenanigans" and even start to exhibit knee-jerk doubt about such things. Often, this is a tragedy, as the issues may well be both real and difficult to discuss.
The quality of such debate and discussion has been hurt by the "Eternal September" nature of online discussion. The internet gives everyone a voice, especially if you have time to waste. Hence, clueless Freshmen and middle-schoolers have disproportionately loud voices online.
Since blood leads, this is, in my personal estimation, done primarily to increase immediate viewer count, not avoid - and thus retain in the more distant future - discomfiting sensitive current viewers.
Yes, but students shouldn't expect that warning before a class on horror films. If you're unable to see graphic imagery, you should drop the class.
Similarly, students shouldn't expect to be warned about a discussion of slavery in a course about the American civil war. And you really shouldn't be able to get a degree in American history if you're unable to study the civil war.