Or not…especially when the magazine in question has a rubber band wrapped entirely around it. There are two reasons why the magazine you want might be being held captive.
First, as in this case, the comic might be very popular (I love you One Piece!) and if people were able to read the next installment, then they probably wouldn’t buy it. A lot of people in Japan don’t really have the space for keeping old magazines or books, so many of them will buy either the book series as it comes out, or the magazines and then sell them off to a second hand store like Book Off. In Book Off however, the books are not bound in plastic like in the books stores, so you’ll see the people who didn’t buy the item brand new standing there and reading the books.
Second, the other possible reason that they might be contianed, is that the magazine might have a present for you! Many magazines will contain plastic bags, make-up, stickers or other items that you would want from the brand / characters that you like. And of course, you need to buy it if you want it. 🙂