Your article also states that:
> However, it is a far from perfect analogue. Bryan Caplan cites evidence that immigrants to the U.S. commit crimes at a far lower rate than U.S. citizens.
So? This is expected! It shows the success of current immigration policies. The current policy is to only let the best of the best into the country (i.e. Those without criminal records and who has at least a degree in a technical field/ or doctor).
The blog post makes no argument that th crime rate of immigrants would still be lower if this barrier and selection would be removed (it would not!).
Furthemore, look at the South African example in this area - illegal immigrants (without work, etc...) turn to crime. Zimbabweans and Mozambiqucans esp. In violent crime. Many Nigerians ar involved in drug crime and other crimessuch as prostitution and human trafficing. Not pretty.
I strongly suspect that if South Africa enforced its border, Mugabe would have been overthrown.
But currently, most of the Zimbabweans that would be responsible for a coup de ta in Zimbabwe (18-40 year old males), are doing manual labour in South Africa.