The socio-economic explanation is valid when you are talking about domestic populations. But when you are talking about immigrants, we already have the data based on education, nationality, etc how immigrants will turn out. We can use this information to make informed decisions on immigration. For example the OP mistakenly referred to the rapists as Mexicans, they were in reality Guatemalan and Honduran, both of these countries are experiencing break downs of society and government(to different extents) and have crime rates more than 10 times other countries. It is reasonable to assume that an uneducated young male from one of these countries has a disproportionate propensity to crime, this should be factored in, when considering letting them in. The same with immigration from some Muslim countries, discussing the causes of Islamic terrorism is a good exercise and also helpful when finding ways to tackle radicalization locally, but when considering immigration, this is only a peripheral point. Another analogy, if you were going home from work, and you knew that street A has many muggings and beatings happen there regularly, you would take the street B home and not ponder over the socio-economics of that street.

are explained away by socio economic differences

This is not relevant to our discussion but this explanation is not entirely true and this is a matter of debate in the criminology community. For many groups Socio-economics alone don't explain away differences in crime.