Honestly, TV Tropes puts it better than I could ever, but here's my take on this.
When people use the "Well, if you don't like it, don't read it!" argument, a kitten dies somewhere. Only in a couple of cases is it ever valid. The vast majority of the time, the "don't like, don't read" argument is used when someone criticizes something a fan adores (or the author goes to defend, usually in fan fiction. I'd hope professional writers have more tact than to try to address criticism in any way other than merely to say "thank you for reading" and move on) because the person is expressing a differing, albeit oftentimes harsh, opinion.
Criticism is good. Being critical is good. Being able to graciously accept different opinions is good. I think people sometimes forget that in order to grow, we have to make mistakes, and everyone makes them. I also think people forget that art is subjective, and, while someone might not like what you create, getting different opinions on anything you create will help you to expand and grow. Even if a piece is technically perfect, people may dislike the content. None of that is bad.
However, this argument comes around from time to time. The first and often best way to refute it is, of course, "Well, how was I going to KNOW I wouldn't like it until I read it?" And, as TV Tropes points out, even if you only partially read something and then stop, it's still perfectly valid to give your opinion on it. Everyone's entitled to their opinion.
I just really hate that particular argument because it strives to not discuss why the person didn't like it, but just shut down the criticism altogether. It's as if that piece of art is so damn precious, the fans (or author) just can't stand to hear a single negative word. That doesn't help anyone, and it's oftentimes insulting.
This is, of course, all in relation to the latest controversy in some circles I run in, and this argument came up. In this particular case, there really wasn't any warnings about the content. If there were, and people still chose to read it, then that's one thing. Without the warnings of graphic content, that's quite another.
Sigh. One thing I'm learning about the indy publishing circles is that there's a new drama every few weeks.