My brother worked in store loss prevention for a time...about 30 years ago now, so perhaps things are different now?? But then, in Iowa, no, they could not forcibly detain them. Verbal only...but it almost always worked. That is, once, ahem, busted
, and told to stay put, they almost always did as commended. Go figure. (I can't recall if this was due to store policy or the limits of the law...but I think it was limited by law.)
It is also true that they would, as a matter of protocol, confront the thief just as they were leaving the cash register AND would have all available personnel at the front of the store. So, I imagine that it did look fairly imposing, with at least a visual blockade.
He had some really intriguing "cases" that he helped solve and the associated stories were a riot to listen to. You had your individuals, or pairs, that would steal just to save the cost of buying, but the real pros were the theft rings that would steal and then sell off their ill gotten gains. They were pretty sophisticated.