Anyone who works in an airline and can explain this?

I don't work in an airline but it's for balance.

-Some airlines have a weight policy put in to protect their workers from overstraining themselves which can result in workers comp/ lawsuit
-I've heard that when you're putting more items in your carry-on you're redistributing weight from the top of the plane to the bottom and changing the plane's center of gravity. (But I don't see how this effects anything with the plane because everything is caused by speed and not weight--if there was nothing on top it would still fly, idk ask a pilot.)

Someone post a picture of a 747 carrying a space shuttle.

Baggage going to the hold is weighted individually. The passenger with his hand luggage has a fixed averaged weight. When the aircraft runs out of "available weight" (=possible load) this trick is used to allow the passenger to take his belongings with him.

Isn't this common sense? You don't put all the weight at the back of the plane where the luggages are. You redistribute the weight. Sheesh.