It’s not theft, it’s fraud which is also illegal.
If I sell you a gold necklace I know is fake and worthless, that’s fraud.
If I run an antique appraisal business and you bring me an antique to value for you, I know it’s priceless but tell you it’s a knockoff and buy it from you for pennies on the dollar, that’s fraud.
If I know some information you do not and I make a trade with you based on that information to my gain, that is not fraud. There are rules that could put me in the "insider trading" target, but there are plenty of situations that don't. I might have done better market research than you did and know that some commodity is about to jump in value or some company is about to become more valuable due to some other activity in the world. Me trading on that information is not fraud, it's smart investing.
You examples are fraud and maybe (?) fraud. I'm not a lawyer so I couldn't say if lying to someone selling you something about it's value is fraud or not. Morally it's wrong, sure. Legally, I have no idea. Both of your examples are, IMO, bad.
It quite literally is fraud, whether you want it to be or not. If you KNOW a stock is worth more or less based on information that you currently have but are withholding from me in a trade, that's fundamentally fraud. If you know a stock is worth more or less based on publicly available information that I'm just too lazy to look up, it isn't.
I find it fascinating the gyrations folks in the financial sector go through to commit what are otherwise cut and dried illegal activities but pretend it's ok "because finance is complicated" or unique or some other such nonsense.
No, it's not. When A buys stocks from B based on knowlegde that A has, A is under no obligation to find out whether B has that knowledge as well and has understood what that knowledge entails to the same degree as A does.
Buying stocks based on non-public or not-yet-publicly-dissiminated knowledge is mostly perfectly legal. Only when such knowledge has been acquired by being an insider to the company, or through personal relations with insiders from that company, does one run into insider trading laws.
You're still literally describing fraud "in the general sense of the word" which is what this is all about. Again, the mental gyrations of trying to change the fundamental meaning of what you're doing "because finance" is silly and annoying.
The only justification anyone can rationally give for why insider trading exists as it does today, is because it's a loophole for lawmakers. It is nearly impossible for you as a private individual to have access to information that will significantly affect a company's stock price without either being an insider to that company, or a lawmaker that knows a law is going to go into effect that the public doesn't know about.
Either way, you making a deal on information that significantly affects the value of an asset without disclosing it to the buyer or seller is fraud, by definition. We aren't talking legally what is or isn't OK, we're talking about what fraud is.
"In law, fraud is intentional deception to secure unfair or unlawful gain, or to deprive a victim of a legal right."
Not sharing information is not deception.
FWIW, I'm in no way in the financial markets and have no personal stake in this conversation other that a dislike for incorrect assertions.
Your premise that it's fraud if I don't tell you every iota of information I have as it relates to some trade is, as a non-lawyer, very wrong. I'm unsure of what moral right you expect you have that would even make that assertion correct.