I will be pretty unhappy if this happens on mine, I agree it looks bad.
I think some people have more acid in their sweat than others and this can affect it. I think they should use a plastic that's resistant to that, it's not an excuse but maybe an explanation.
I wash my hands before working on the keyboard and I try to keep them clean throughout the day. There are alot of variables, do you smoke while you type, do you eat, drink, etc. Just some things to think about. Also do you clean the keys? I used to use Windex on everything, IBM keyboards always stood up to it but my POS brand-X keyboards sometimes didn't look so great. I always prefer white but even those can be affected, although it looks worse on black for some reason.
Edit: you're already ahead of me, microfiber and water sounds about as gentle as you can get. I can't imagine that causing a problem. Sounds like you take good care of your equipment.