you sure about that? if they enforce bag weight they won't need to put max weight stickers in the actual plane. It's not like anyone would humor them if there were anyways..
Limiting the carry on weight I can understand. The overhead compartment can only hold a certain amount of weight. So a limit has to exist.
And there is the danger of luggage falling on people during an emergency or even just when opening the compartment. And while 7kg hurts, more weight hurts more and even will get dangerous.
If they put the limit at 8kg you would make the same argument about 9.
I'm not saying they're not rent seeking, I trust them as far as I could throw them, but this is a false comparison. The weight limit of the overhead compartment and the total passenger weight of the plane are two completely different things.