Basically they're so bad they should be crucified.
Some are perhaps but all aren't necessarily. Could be that there is a stockpile of codes for a given game due to it being excessively bundled beyond demand so people have lots of spare keys to donate, or something that was heavily given away as a free promo with lots of keys floating around.
The ratings on some of them are quite high on Steam.
That too, yes.
System Shock 2, for example, got crucified while it was here due to the fact that it was given away numerous times on GOG, to the point that moonshineshadow STILL has 7 copies on her giveaway. I figured if nobody wanted the GOG copy, even less people would want the Steam version. I was, quite surprisingly, wrong. But hey, at least I managed to give it to somebody.