Why is it a "right wing myth"? Far-right perhaps but I don't see the political salience for people with mainstream views (unless you are tying it to present day critiques of Western gov use of force, which often come from the left)

I think a version of it is essentially a form of "common knowledge" that seeped into public memory decades ago. I think its much more likely that people acquire this view when they're assigned slaughter house five in high school instead of encountering it via aggressive apologetics. I'd wager that people generally roll with it in part because it serves as a nice "visualization" of an intuitively moral philosophy 101 bullshitting scenario (along with Coventry and the "24-style terrorist ticking time bomb").