You can’t freely sell devices to let others self-diagnose medical issues, so this part of your analogy doesn’t hold up in the case of phone sales.

We also limit investing in certain types of investments to so-called “accredited investors” which is just legal jargon for “millionaires”.

I don’t think the point you are trying to make about letting people own-goal is as strong as you think it is. (I would have gone with “roulette is legal”, which is a better one that the investment one, as the accredited investor rule is in all 50 states.)

I'm not certain what you're advocating for here?

If you are interested in the public good, I think it is pretty clear that we should ban roulette overnight since it has a negative expected value for everyone but the casino. On the other hand (still presuming you're interested in the public good), I think you have to consider very carefully whether it's good or bad to lock people out of investments or to restrict people's access to health care.