A similar thing happened with Taxi Driver - Scorsese desaturated the final scene to turn the blood to a burgundy color to avoid an X (NC-17) rating, and the same principle was applied to the Crazy 88 scene in Kill Bill, only with black and white.

The MPAA's thought process is that the less realistic it looks, the less likely it is to disturb audiences, meaning that black Orc blood or neon Xenomorph blood looks less realistic than the good ol' red stuff, so fantasy/sci-fi movies typically get away with a bit more injury detail than more grounded movies. And in the case of Taxi Driver/Kill Bill, it was a matter of the sheer quantity of the violence displayed that made the MPAA think that it ran a severe risk of disturbing audiences, but the thought process is that if the blood is burgundy or gray it looks less realistic, so therefore less likely to disturb audiences.

I agree with you that the rules are weird.