>> we would have to assume that any group with a high arrest rate must be a group that commits many crimes

That doesn’t hold - there could be more police near a group of people who commit the same number of crimes but because there are more police resources, there would be more arrests. There are lots of reasons this doesn’t hold.

>> saying they're less likely to commit crimes is a speculative inference

It is also undisputed though. We’ve always known illegal immigrants avoid making the kind of trouble that risks run ins with the law (i.e. there are far fewer “safe” crimes for them to commit) because the penalties are deportation.