“The bill of rights constrains what the government, not what private enterprises, can do.” This is untrue. The 13th Amendment states you cannot be taken as a slave. If your assertion were true, Costco (Kmart, et al) could take you as a slave. Obviously, the Bill of Rights applies to protections of the individual American citizen. The 4th Amendment does not limit or qualify what its words apply to. It does not say only government, an agency, or business. The 4th amendment says what it says.
http://www.senate.gov/civics/constitution_item/constitution.htm is a reference. The explanation provided in the Senate link for the 4th Amendment is:
“Applying to arrests and to searches of persons, homes, and other private places, this amendment requires a warrant, thereby placing a neutral magistrate between the police and the citizen.” Costco et al are private places. I can’t just search you because you may be on my property.
Further, who decides what “unreasonable” means? My policy is that it is unreasonable to be detained by Costco et al, for the purpose of their agent searching my personal effects without warrant or probable cause (which would stand up in the court case I would bring). I am an American citizen covered by this Amendment and fully capable of determining what “unreasonable” means. Costco et al would have to prove in court that is reasonable for it to detain and search all its customers before allowing them to exit its premises. I’d like to see Costco et al drawn into just such a case in court.
The 13th Amendment isn't part of the Bill of Rights.