What would you do to prevent it?
Public ownership of shares of stock is a natural effect and expression of property rights. If someone owns a business, they can sell shares of it to whomever they want, including the public at large. I would suggest to you that most “private” companies are the same. In fact, any form of business other than a sole proprietorship almost certainly is divided into “shares” that have been exchanged in various ways at one point or another. A “private” firm most likely has some shares held by the owner, some by a family perhaps, some by a private equity fund, etc.