But again, what property is being taken? Unless you consider “the right to buy stocks” as property itself, which seems tenuous. You basically could apply that to any such item: “the right to buy drugs”, “the right to buy nuclear weapons”, etc. This isn’t a value judgement on whether or not one should be able to buy prohibited items. Rather, it’s an observation that clearly the legal system does not interpret “the right to buy” as property covered by the constitution.

By the way, society does generally let you buy whatever you want. It’s only when your ownership of an item infringes the rights of others are you generally barred from owning something. E.g. you’re not allowed to buy nuclear weapons because they pose a danger to those around you. Allowing policy makers and their families to trade stocks also poses a danger that insider knowledge will be used for personal gain in the market. This is a form of theft against the market and public at large. What about the rights of everyone else?