I know this guy has been since given the boot, but I thought I'd give my .02 on this very topic.
Nobody here is denying the conditions at those facilities are dire. It is possible to detain people humanely and authorities are reneging on this responsibility to varying degrees.
The simple fact is that most citizens of any country do not approve of the idea that an unlimited number of people can cross their borders undocumented and face no consequences. They should expect the sovereign nation to enforce its laws, albeit in a humane fashion. Even from a libertarian standpoint (notice the small "l"), you seem to prioritize the rights of non-citizens over the rights of citizens. Progressives think the criminal threat posed by illegal immigrants is overblown while conservatives believe their presence threatens the very integrity of the country's demography, but what's undeniable is that the illegals' collective choice to cross the border with no documentation both poses a threat and undermines the rule of law.
I speculate most people support detainment centers out of necessity rather than enthusiasm. Few people have the stomach to see people herded like cattle even if they're breaking the law, but if the options are to let them enter America without any identification, turn them away at the border by force, or to detain them somewhere relatively comfortable while their claim to asylum is examined, then anyone who cares about the borders will choose the latter.