The ratings system is anonymous so players you give a red thumb to won't retaliate. I don't think it's fair if paying members get to see who gave them a red thumb. Premium members will end up getting less red thumbs, even if they really deserved them.
We've had a lot of discussions on this forum about red thumbs. I personally think you should have to provide an explanation for why you gave someone a red thumb. Some players give red thumbs for petty reasons - if they lose, if they don't like a move you made, or if you were nice to a beginner and tried to explain how to play the game. You're also more likely to get red thumbs if you play multiplayer cooperative games like Saboteur and Hanabi.
I don't give out red thumbs that often, but when I do, it's because the player was rude or cheated. Because I hate that type of behavior, I usually end up telling the person exactly why I red thumbed them. I wish people I red thumbed weren't allowed to join my tables or show up on the "available players" list.