In my case, checking into a Cathay Pacific flight (7kg carryon limit), they insisted on weighing my bag, found it to be around 8kg, but let me carry it on anyway.

I would not be surprised if the rule is there to avoid injury to flight attendants. Even if the flight attendants are not supposed to lift carryon bags into the overheads, passengers will probably ask for help anyway. Having a carryon weight limit probably helps avoid flight attendants being forced to either risk injury by lifting a heavy bag or give the impression of poor customer service by refusing to help the passenger.

Best policy for the passenger is:

a. Your carryon bag should be something that you personally (or persons traveling with you or assisting you) can put in the overhead or underseat space without assistance from flight attendants.

b. If you know that your carryon bag is overweight, pack the most essential items in a smaller bag that is under the limit so that you can remove the smaller bag and carry it on if forced to check the overweight bag.