It's my fault. One brave group of survivors caught some marauders as they were making one such grisly pile. They were killing their captives one by one, bashing their skulls in and throwing the bodies into t ditch on the side of the road. The marauders were subdued and...questioned.
The answer was simple: all along the lines of communication between the marauders, they're spreading the word that a group of people are killing men and women they find with captives. Anyone that looks like a marauder is treated as such.
I don't know how many of them read this blog, but I'm sure some do. A lot of them don't have access to wireless communication, so they use etched symbols in places other marauders pass through often. Kind of like hobo codes. There's one that basically means, "Danger, eliminate captives". Once prisoners have been disposed of, a group of marauders looks like any other set of travelers.
Hundreds of dead across the US because those men and women are worried some vigilante group is after them. This is too much. I can't even think straight right now.