First of all, readability of something is going to vary person to person. Studies may have found the averages skew toward less readable, but I doubt that applies to 100% of the population. Once you are used to it, it will hardly make a difference.

Secondly, it's only objectively bad in your contrived scenario where the only thing taken into account is readability.

I have been using a cursive font for terms like "public function" and other oft repeated phrases. The stark difference allows me to more quickly skip over them. I don't need to 'read' them.

But my main reason is I just like the style. I stare at code all day long. I want to make it a bit stylistic and fun. Lots of people also have backgrounds on their desktop, but a solid black background would probably be less distracting and make finding icons easier.

I also wash and polish my car on occasion, even though leaving it dirty would not affect it's primary purpose, to carry me from point a to point b.