That's the law. The reason is, you can be a passive investor in a company, similar to buying stock in a company. This is allowed on any type of visa including H1B. But, if you are taking an active role, you are working for your own company and you cannot work for a company without work authorization. The work authorization could be CPT, OPT, EAD or H1B ( there might be other forms of work authorization I am only aware of these few).