Do you mean that denying entry is illegal, or denying exit is? Because these unmanned stores do let you leave; they just have separate entry and exit doors, wh

Do you mean that denying entry is illegal, or denying exit is? Because these unmanned stores do let you leave; they just have separate entry and exit doors, which have mutual-exclusion locking (like an airlock) such that nobody can enter the store through the exit door by getting someone inside the store to open the inner door. (You could slip into the little individual exit hall through the outer exit door; but the inner door in the little exit hall would remain locked until you left and it detected there were no longer people in the hall. Presuming a high-traffic store, this also wouldn't hold anything up, as other customers would continue to exit through the bank of other exit halls to either side of you.) I haven't checked, but presumably pulling a fire alarm inside the store would disable all this security, flinging both the entry and exit doors open. But you can't reach the fire alarm in the exit hall; it's not "inside" the store. The only thing you can do in the exit hall, is exit (which you are always free to do.)