I believe we agree to disagree. You simply refuse to accept that you are not entitled "to carry any single piece of handluggage you want" but "a piece of handluggage which does not exceed eight kilogramms". To get your piece of luggage on board at the gate you need to make evidence that you are within the rules. You demand something, so need to prove your entitlement. This appears to be the one lesson you have missed on law school or have not yet attended. If your are within the limits, good for you. If you are within the limits and they deny boarding, you have a case. If they do not check, even better because it avoids trouble.