When I purchase a stock, I do it in the express belief that I will get a physical (though digitally stored) share of that company, and possibly one that gives me a voting right if the stock is marked as such. When a stock “fails to clear” this gives me a ton of problems such as slippage and volatility, and possibly a quite substantial loss.
Same if you buy an apple, I'm sure you'd only do it in the belief that you will actually get something edible. If I got nothing, I'd want my money back! But then when you buy an apple, you can see and touch it before you commit. Not so with stocks. And so you buy it while trusting the broker that your order will actually be met.
If instead my money is “borrowed” without my concent for some nefarious activity—in order to create more “liquidity”—that has a name: It's fraud. It's fraud of the customer whose money is being stolen. It's fraud of the customer who's being fooled into thinking that he's buying a real stock. That these shares do not exist, isn't some slip-up. It's an intentional effort, done with the motive of earning money by exploiting the trust of their customers. Such action should thus clearly be illegal.
Same if you bought a stock, and your broker suddenly decides to steal it without your knowledge, and loan it out in order to sell it in the hopes of earning money if its value drops (i.e. short the stock). Clearly you'd want to know if your property is being loanded out, because it means that you're incurring risk, no matter if you're compensated for it through interest or not.