They're going to look stupid in the end anyway, whether you let them yank you around or not.
And even if my leaving furthers the assumption that I'm guilty, I don't care about assumptions. I care about the fact that I'm not a thief and I'm not going to let anyone treat me like one
also, I come from the angle of knowing that plenty of this kind of thing is based less on the presumption that you're up to no good than on a desire to pick on, bully and "dominate". Not all merchants who eye you like you're trouble really think you're trouble for example. Some just want to insult you.
Nobody has the right to detain anyone based on bullshit, so with me, it ain't happening. Besides, what happens after you let them "have their way with you" and they find out you weren't stealing? A half-hearted apology with them walking away feeling big and strong (and ready to do the same thing to your kids) and you walking away feeling what...?
No thanks. Meet the devil halfway, and you're just that much closer to hell. You'll end up having to meet him halfway yet again
Instead, you should always stand up for your dignity and always fight back.