It is up to the author of an app how it is distributed and who is allowed to distribute it. Even free apps should not be redistributed without the developer's agreement. Of course many developers may be happy for people to distribute them, but if they have not explicitly said so then it is an infringement of copyright to do so. That's a general principle, and even if you can see no good reason why the copyright holder should not permit sharing that is still their choice to make.
The difference with Open Source is that you have a license which explicitly permits sharing, so the author has given you permission.
Now in this particular case there was a suspicion that the reason you couldn't download it from the store was that Samsung did not want it distributed in your country. If that is so (which we don't know for sure, but let's assume so for the sake of illustrating the point) then it would be very clear that someone sharing it with you another way would be doing so against the wishes of the developer. And since you put emphasis on the app being "free", I'll note that your reason for wanting it was to get free access to books that you would otherwise have had to pay for (or, of course, not download). Therefore in this specific case it's not true that sharing the app has no financial implications for anyone else.
Not trying to get on your case here, just pointing out that this stuff isn't as simple and one-sided as "they are not charging for the app so what's the problem?"