Sounds to me like a joke that is usually used when the identity of the perp. is is not available.
It's an unfortunate running "in-joke" here on FR, that when a news story does not identify the ethnicity or race of the perp, some folks assume it's a typically black thug, and so they use "Amish" (presumably non-black, non-thug) to call attention to the lack of specificity of the news report, and to point up the assumption.
For some of us, the "Amish" in-joke has long since run its course. It's been particularly unfortunate in those very few cases where, in fact, a perp WAS Amish. But it lingers...