Why do adults grab thing out of kids hands? You wouldn't do that to any other person, would you?

I think it's just usually a quick response to avoid a possibly dangerous situation. Or an embarrasing situation if there are others looking at what the child is doing. Like you said, the best way would probably be explaining it to them before grabbing it out of their hand (or after doing so). But a busy mum maybe trying to do several things at once - or have already told them not to do something many times before, so they don't have the patience to explain it again or act more assertively at the time when their child picks up something.

This is done through pure fear that the child will hurt themselves, and maybe a little guilt on the parents part for allowing a child to hold anything that is harmful to them. By grabbing the object and scaring the child slightly, might it not be the best reminder to them that they are doing wrong. Take the object away, then explain why you did it, and how it could harm them. Short, sharp, shock is a good way of stopping them from repeating the wrong they may have done. It's not being rude to them, or in any way treating them any different as you would an adult, because you would expect an adult to know what they are holding is dangerous. A child has to learn, and be taught these things.

if the child has something dangerous then i would grab it out of their hands, but i also would explain to them why.

it depends on what your child is holding! if they have something that could hurt them, then of course you need to take it and explain why you are taking it. A child is still learning. they need to learn what is good and bad, a parent needs to do this if a child doesnt catch the concept of, yes, no, share, and so on. of course you wouldnt take something from someone else, especially if your not meant to. but thats the differance. some things are nessisary to teach the rights and wrongs in life! i hope you understand what i mean!