[pierstroll]

They have a store policy that you show the receipt.

Are you saying they have no right to have that policy or to enforce it? Are you saying that a business or private citizen is held to the same standard as governmental agents when protecting their property rights?

I will guess that you don't own a retail business.

[Quick1]

They have a store policy that you show the receipt.

Having a store policy doesn't make it legal. The store policy could be a cavity search on your way out (after all, you can hide that flash drive anywhere, you little thief!). Would that be any more legal?

Are you saying they have no right to have that policy or to enforce it?

If it's an illegal policy, then yes, they have no right to enforce it.