Because they're using their pinball machines as decorations.

I'll bet most of these machines awesome from about ten feet away.

This is like a bit from a comedy movie.

"Do you like pinball? Well this machine is FULL of pinballs! "
[Sight-gag of pinball machine so completely full of balls that none of them can even move.]

"Uh, wow, that's sure is a lot of pinballs."
"It sure is! Say, you want coffee?"
"Sure. [thinks a second]... uh, no wait!"
[Firehose.]

It's a principal of science that illuminating multi-colored printed art with monochromatic light will make it hard to see the art.
If you don't agree with that, you're not stating an opinion, you're just wrong on a point of fact.

So the question is : Is it a good thing to make the printed art look less visible for the sake of a (allegedly) cool glow?

Maybe if they hated the original artwork. Maybe they were thinking "Wow, the coloring job on this translite is awful. How can I hide it?"