If a link does not set target="_blank", you can always choose how it will be opened: in this tab, or in a new one, with a left or middle click.
If it has target="_blank", it'll always open in a new tab, even if you don't want it to, and there's no visual indicator that this parameter has been set.
I never set target="_blank" as a general rule for this reason.
That's the wheel, for you guys coming from a two-button-mouse Operating System.
We never reached an agreement but someone did make a claim, that I was unable to verify, that modern users don’t use the back button and therefore relying on it to allow users to find their way back is unacceptable. I wonder if anyone else has heard that (I suspect the HN crowd knows about the back button though ;))
Also, CTRL and left arrow will return you back to the HN. It's not that big of a deal? Also, it's a good thing as it teaches you some good habits. Why depend on a site to open links in new tabs? Do it yourself. That way you force consistent behavior across all websites/links.
Why do you want to remove my choice in the matter?
Each method is dependent upon various things including, how I'm sitting, number of items I plan to open, number of comment sections vs just article link, and most importantly, how sore my hands are. Some methods are faster but strain my wrist more than I care to do, so I do those sparingly.
One of the reasons I love this site is the UI, it's very simple and gets out of the way for you. If you want to change the opening mechanism to only do new tabs, you are completely able to do so with code.
To answer your question, there are a variety of reasons why HN may choose to do it this way, other sites have different reasons so you see different site behavior.
However, if it helps:
- Control + Click OR Mouse middle-click