I would never defend the US's use of torture, suspension of habeas corpus for terror suspects, etc. They're horribly corrosive and embarassing things that every American should be ashamed of.
But, our "secret police," if you will, is the CIA, which only operates abroad. The whole reason that Guantanamo Bay exists is that the CIA and military were afraid of having to be subject to US civil law and courts, so they held all the illegally captured and held prisoners on a base that's on foreign soil.
There's really no risk to someone who is vacationing in the US of being arrested and held without warrant. If the authorities thought that you were dangerous, they wouldn't have let you in, hence the ridiculously onerous requirements for getting in, even for visa-waiver countries. The FBI and domestic police know better than to try to flagrantly break laws regarding habeas corpus or torture, because if it happened within our borders -- where our civil courts have clear authority -- they know their asses would be handed to them.