Every culture has some level of commonly accepted paternalistic policies. Forcing people to wear seatbelts, banning certain drugs etc.
Not allowing small investors to buy shares in non-public (and therefore not needing to disclose much information) companies doesn't sound like an especially radical idea. You just need resources to do due diligence to invest in such companies and as a small investor you don't. There is a common interests in preventing people from being idiots in the investment world.
First, just because people do it doesn't mean it 'should' be done. See the naturalistic fallacy.
Second, just because smaller investors lack resources, doesn't imply they should be prevented from acting with less information. This doesn't follow from your final claim that there is a common interest in preventing them from participating and this is unsubstantiated.
Let people make their own mistakes, you don't get to decide for them.
Hmm, I'm struggling to find one that you might think would warrant external rules to restrict someone's freedom to make a mistake. I'll think about it more.
Not allowing small investors to buy shares in non-public (and therefore not needing to disclose much information) companies doesn't sound like an especially radical idea. You just need resources to do due diligence to invest in such companies and as a small investor you don't. There is a common interests in preventing people from being idiots in the investment world.