You were not penalized, your opponent lost that end, you have won that end.
The overall match result caused your rating to drop.
The system is not perfect, since we cannot put an umpire to each match, who can decide what would be the correct score when someone leaves early.
One thing is for sure: the one who quits looses the end (not necessarily the whole match). You can debate the resulting score of every such end, but the wisest thing is to accept the system as is, unless you can propose a better one, not for just one specific end, but an overall system which is valid and fair under all circumstances.
OP