Another way to look at it is that immigrant groups bring parts of their old world with them; German-Americans left a high-social-trust, low-crime culture and established it in their new country.
Did they? I can't speak to the crime rate of the German empire, but a very common reason to immigrate, in addition to availability of farm land, was to avoid the draft (low social trust). The slow rate of assimilation is also a signal that these were not people coming from a "high-social-trust" background.
But, to add to and enforce OP's point, their cultures were a larger schism away and they instead held on and entrenched their identities (to their objective detriment, no matter your moral stance). Meanwhile, Anglo, Franco, German, Dutch, etc cultures all kind of melded into the early "White American" identity; due to the general ease of assimilation.
I'm not sure that's true. For example, in the United States, it took numerous generations of German-Americans to fully integrate into society, with towns in Wisconsin speaking a dialect of German well into the 1940s. Despite this lack of cultural integration, these cities experienced very little crime.