Shopping bag searches
Many traders have a policy of searching customers’ shopping bags to reduce the incidence of shop lifting.
When you enter a store you do so under an invitation of the seller and the seller can make that entry conditional. This is what happens when stores display signs that state shopping bags will be checked.
You can refuse to allow a bag check, but in these situations the seller can ask you to leave the store and not come back unless you agree to the store's condition of entry.
They can also refuse to sell you any goods and they may call the Police if they suspect that you have committed an offence.
If you do not agree with the store policy, then you should not enter the store.