I think more people have issues with the size and amount (by extension) of carry on more than the weight.
The allowance is quite commonly one piece per passenger measuring 105 cm or 115 cm linear dimension sum; sometimes two pieces. Personal items - such as "reasonable reading material", a laptop or portable device, a handbag or the like - also seems to be acceptable. Things which don't seem to "count" for "unconditional" reasons include photography equipment, musical instruments and duty free items purchased on airside (i.e. after check-in procedures).
As long as one sticks to these limits, I wouldn't have thought people really cared much about the weight, provided one can lift their own carry on items. In the USA, if you hurt someone due to your bag being really heavy, either way a lawsuit will likely be coming your way. The difference between thwomping someone with a bag of 7 kg versus 27 kg will only be reflected by their injuries, and thus how much you'd better hope you have pockets lined with money, or have access to a good attorney. The probability in which you might have recourse to litigate against the carrier for their being complicit in letting you carry something which was (read: turned out to be) "dangerously heavy" on board is questionable.