#4
I'm a law student and I talked to my prof about that because it bugs me too. He said that it falls under shopkeepers privilege. Wikipedia it.
By not stopping to show our receipt you give the store reasonable suspicion that you've committed a theft. They are then allowed to detain you for a reasonable amount of time to investigate weather you have stolen anything.
They might let you walk out without checking your receipt most of the time but if they really want they can detain you and then check your receipt to make sure you paid for everything.