The practical current function of commercial space co's should be to provide routine transfer of staff and supplies to and from a station or base. That makes perfectly good sense. When something becomes a semi-commodity, private enterprise, with competition*, is usually more efficient.
If and when space does become profitable, such as asteroid mining, such commercial co's will already have some of the infrastructure and knowledge to pursue that market.
As far as pie-in-sky commercial endeavors like a one-way Mars mission, let investors waste money if they want. Who knows, maybe they'll stumble on an unforeseen way to make a profit. Surprises happen. If somebody discovers how to tame anti-gravity particles to get cheap launches, for example, existing space companies will have a big leg up. It's not irrational to devote some of one's investment portfolio on high-risk/high-reward stocks.
And even if they fail, humanity as a whole will be smarter for it, learning from their mistakes. Failure is experience.
* NASA does use lots of private contractors for current missions. But, they are mostly custom one-off products.