To keep this as straightforward as possible: Illegal immigration is a crime and is enforced just like any other. Committing crime can mean incarceration, which many US citizens face everyday. It has nothing to do with being a "danger", but when you don't have permission to even be in the country then there's no bail or other release possible.

Also detainment centers are not cages but fenced areas with free movement inside where people receive food, shelter, healthcare, entertainment, schooling and legal services paid for by US taxpayers while their cases are processed. Detainees are free to deport themselves at any time. This is more accommodating than pretty much every other developed nation.