> What I don't get about this is why trigger warnings and safe spaces always seem to get lumped together in discussions like this.
Because they are involve quite similar problems (safe for whom and from what? Warnings for which specific of the billions of potential PTSD triggers?) and are typically advocated for together, by the same people, as linked demands, to serve similar broad purposes.
> But trigger warnings are a really simple common courtesy.
No, they aren't. They are neither simple, nor are they, in fact, a common courtesy (whether or not you think they should be a common courtesy.)
> There was a popular meme spread around the Fourth of July to be considerate of veterans who may be distressed by the sound of fireworks.
There were actual several different ones, most of which were variations on combat veterans with signs at their homes asking for consideration and restraint.
> If you think this is a consideration worth extending to a veteran, it seems perfectly consistent that you'd want survivors of rape or abuse to know that a discussion of rape or abuse was coming up.
(1) Expecting people to, proactively and without knowledge of the particular trauma experienced by or triggers relevant to the audience, provide "trigger warnings" is exactly the reverse of the kind of specific and personal request for consideration at issue in the meme, and
(2) Triggering with PTSD is, in fact, quite complicated, but most often (AFAIK) involves sensory stimuli that are associated with the instance(s) of trauma producing the condition, such as sights, sounds, smells, etc. that were present during the trauma. Discussion of the class of event that relates to the trauma may be areas of strong emotional context and political sensitivity to many people (whether or not they are victims of that particular kind of trauma), but nothing I've read of actual PTSD and triggering suggests that they are particular likely to be triggers in the strict sense, even to victims of the trauma under discussion -- even to victims that do have PTSD and do have genuine, identifiable triggers.
(3) Proactive trigger warnings not based on the real known specific triggers applicable to a specific audience, if one isn't to give priority to certain type of traumas and certain presumed triggers associated with that trauma as "more deserving", quickly turn everything into a mass of warnings, most of which are irrelevant even to people who have PTSD originating in the type of trauma the warning is focused on.