You are a member of society. Society stops people doing harmful things to themselves all the time.
Because, you know, certain actions and even thoughts can lead to eternal damnation in Hell, according to what a society may think. Would you prefer the society to hold you off from that?
If you see a child playing with a loaded gun, you won’t stop it?
Money, in any form, may be as dangerous as a loaded gun, trading stocks or not. Most adults are careful with money, as they are with loaded guns. The problem is that some parties may try to make trading stocks (even leveraged) look much easier and safer than it is. It's like giving somebody a real loaded gun, while making it look like a toy gun, safe even for a child. And this of course needs to be regulated: not the trading, but the disclosure. This is not a toy.