Chess.com Tactics Trainer Rating System

Tactics Trainer rating system is the worst. They give you only 8 points if you get 100% and take 9 points away from if you failed, why is that? And not only this, but you might solve a puzzle correctly, then you don't get anything or even some points are taken from you if you took more than the average time trying to solve the puzzle. Why don't they change this system? Can anyone explain to me how does it work?

Not worth worrying about.  The new version which is available over on chess.com V3 (premium only until they finish it) is much improved.  I hate playing devil's advocate here since I have my own issues with the TT, but the idea is not to have you "calculate" a solution, but to instead "recognize" it through the magic of pattern recognition.  Nah, I don't buy it completely either, but that's the idea.  It's actually quite a concession to critics that the new TT no longer penalizes you for taking too long.  You may not get any points if you take a long time, but you won't get any taken off either.