Honestly, everything could trigger some group based on that logic. What about a zombie pictured with missing limbs at the local recreation center triggering amputees. What about a nerf whistling football at a school setting off refugees who'd escaped bombing in their home country.

Why does this scenario warrant action, when other similar situations do not?

There's a line that needs to be drawn somewhere, and I honestly don't think this sign is on the wrong side of it.