Thanks for the welcome and the replies!
The theory that the strip prevents employees from stealing books actually makes sense. It's still weird though, because I've been buying things from them for the past 5 years and have never seen anything like that before. On the other hand, they do sell a lot of foreign books (like the one we're talking about), and the case might be that they buy them from different sellers and some of those sellers implement anti-theft practices like this.
As for me, I'm not too much of a reader actually. I was puzzled by this thing and so I needed to ask somebody about it. I knew that I would need to have a photo storage account if I wanted to ask a question on Yahoo! Answers or a similar service, so I decided to try asking on some sort of a book forum - this one came up first in a Google search, so I registered. Then I discovered I couldn't post attachments, which meant I needed to get a Photobucket account anyway. :/ And then I discovered I couldn't post links either, so going back to the original Yahoo! Answers idea seemed to be the only option. And then I also posted the question here, hoping to receive more answers and that the moderators would not delete the thread because of the link included.
So yeah, I don't read much, and when I do it's usually something in English as I'm seeking to improve my language skills.
The book in question is Yukio Mishima's "Forbidden Colors". It may seem strange that a person would try to improve their English by reading translations of Japanese literature, but I just happen to read all kinds of translations into English recently without reading anything originally written in English. I don't know why, really. The only exception are the writings of Truman Capote and Kazuo Ishiguro. Well, not Ishiguro, since I had to read him in Russian (which is my native language), as I couldn't find the book I was interested in in English.
Anyways, if anybody has any ideas about the weird strip in the book that haven't been mentioned yet, I'll be glad to hear (read) and consider them all. The "anti-theft device againt crooked employees" theory seems to be the most plausible so far.