Well, the idea is to detain families together pending a hearing to vet their credentials and determine their status. The ultimate issue is what our policy should be toward granting asylum to families fleeing intolerable gang violence in Central America. I know my views on that score differ from most here.
Under U.S. law, asylum can only be granted if persecution is “on account of race, religion, nationality, membership in a particular social group, or political opinion.†Being subjected to gang violence from drug cartels almost surely doesn't qualify under U.S. law, which is why the vast majority have been rejected.
If you want that expanded to include being subject just to a drug-related high crime rate, the law would have to be changed. And tens of millions of people would qualify for asylum under that standard. Arguably, a great many more than that.