If that's the case, why do Pattern Day Trader rules exist? They seem to generally cover "protecting you from yourself", not from some third-party.

I don't think this is true. A clear example is startup stock-options. It is legal for the company to give you that as compensation, you historically get taxed for that compensation, it is legally mandated to get appraisal by a third-party company, but it is generally illegal to offer them for sale in the public.