You are correct that the Fourth is a stricture on the government, but tort law applies a similar correction to private entities that conduct 'unreasonable' searches and seizures. Let's say they decide they think you stole something, but did not witness you steal it.
(snip)
I know a lot of cops think store owners can pretty much do what they want to customers - they can't. The rules of probable cause apply to them just like anyone else.
Except...cops don't enforce tort law.