That's a very good question. I know that because I asked it myself. Well, it's very simple really: I don't think you're stupid! Modern browsers have many ways to open things in a new window. Here are a few,
However, as far as I am aware, there is no way to override when the link you click is designed to open in a new window. I have chosen to give people control over how they view the site, rather than taking it away from them. If you want to open in a new window, you can. If you would like to keep using your current window, you can. I'm sure you all know what the "back" button on your browser does too, if you would like to change your mind about a choice you made to open in the curent window.
Some of you may be wondering what the difference is between background and foreground in this context. It means that the new page/tab/window doesn't steal focus. Normally when you open a link in a new window that window pops to the front, stopping you doing what you were doing until you choose what to do with the new window. If you open it in the background you stay in control of the page you're on while the new page loads. You can then choose to look at the new page when you're done with the page you're on.
This is helpful if you want to open the contents of links relating to an article you're reading without having to stop reading to get rid of new windows or to have to hunt through the article for the links once you've finished reading. Also, if a site has a large array of thumbnail pictures you can click on several in a row that you would like to see larger, without switching between windows continuously. Middle clicking is your friend.