That's actually Hacker News being terrible. Their font declaration is just "font-family: Verdana;" so if you don't have Verdana installed (which just about never is installed by default on Linux), it defaults back to the one mentioned in the browser settings (which tends to be a serif font).

Hacker News could've made their font declaration have a fallback (like "font-family: Verdana, sans-serif;") and things would look way better by default.

As for me, I solved it by editing userContent.css and adding a font-declaration for Hacker News myself. (or you could use something like the Stylish addon)

All in all, I'd say it's the fault of website creators.