I will ask the same question that I asked Hawk.

As a customer, you enter a store under licence.  Retailers are legally able to set conditions of entry. Those conditions need to be displayed in a way that are clearly visible.  They should state that bags over certain size that could reasonably be expected to conceal goods, may be checked.

Why do you feel the conditions do not apply to you?

Why would you put yourself in a position where a shopkeeper  could terminate the licence agreement as a consequence of your non-compliance?

I have worked in a position where I have had to handle such disputes and I know retailers are within their rights to refuse future entries.   It is such a minor inconvenience to comply and such strong objectionable  attitudes are so unnecessary IMO.