Maybe it's the same reasoning behind having 4:3 content blown up to 16:9 - people arguing "I bought a color display handheld, so I want even monochrome games being colored by default".
The Super GameBoy had some nice default color palletes built in, e.g. try Tetris and Super Mario Land once if you like to.
However, I agree with you about what the monochrome games look best. For my taste, the monochrome pallete of the GBC/GBA/GBP has a bit too much contrast for my taste, so light grey and dark grey look too much alike white and black. I'd have welcomed a more softer palette or even one that's emulating the yellowish look of the original GameBoy.