As far as I know, the SEC hasn't banned the participation of US citizens in the ICOs outright. They have just expressed "concern" over the rising number of ICOs and the shady people behind them. So unless the ICO explicitly bans US passport holders in its terms and conditions, you can participate in them. The federal income tax will be 39.6%, if your annual income is more than $418,401.
So there is really no penalty to be imposed by the government to someone who covertly joined an ICO even if the company doing the offer is clearly not accepting US citizens. Anyway, in this case, it is not anymore the fault of the ICO issuer but of the ICO buyer but they have to declare it once there is already a gain out of the transaction and must pay the right rate of tax. That announcement from SEC on ICO is just an introduction of the many possibilities in the future and definitely it was not detailed.