I think it is $$$ too!
If it were safety, then there should be a standard, applicable across all classes. There isn't, so until there is, the airlines with 5kg limits (some of the Brit carriers) will be way down my list of people I want to fly with! Consider this, you can leave the same British airport on one carrier with a 5kg limit, and another with no weight limit at all (you must be able to lift it, I believe is the restriction of weight). Seems utterly silly to me.