To me, a classic "tontine" pays out nothing until the next-to-the-last death, and then gives everything to the sole survivor. That doesn't seem very useful to retirees.
Milevsky talks about a series on one-year "tontines" in which all the funds are paid every year, regardless oft he number of deaths. This type of "tontine" seems useful to me. As he points out, it is closely related to a payout annuity, which is certainly legal.
You could do that, but the one-year variety would only work for sudden deaths, as people would drop out after deteriorating health is discovered. A tontine can pay out dividends and some agreed-on percentage of capital each year (like an annuity) and then liquidate after 50 years, for example. You could avoid the investment restrictions, overhead, and risk premiums of insurance companies and still hedge some of your longevity risk. Interestingly, on Wikipedia there's a comment that they operate "underground" today. It also mentions JTWRS, but I think you'd have estate-tax issues there.