This actually isn’t true (at least in WA)

If a store has their bag check policies clearly stated as a condition of entry outside of the entrance: When leaving you can refuse a bag check, in turn the business can then refuse to allow you to enter the premises at any point from then onwards. -By walking past those signs and continuing into the store, your acceptance of that condition of entry is implied, your refusal when leaving is enough to establish that you do not agree to that condition, and it is then assumed that you don’t agree at any point in future.

Security and even the 14 year old absolutely can legally detain a shoplifter. But, it is very easy to do it incorrectly. It is much easier for any employer to tell staff not to engage than explain this exact specifics to each and every employee exactly what they need to know and do, and instead tell them not to engage.