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.
I wouldnt paint this so cut and dry. I will agree that they will not retroactively enforce this; that is making a list of participants and going after them (would be really difficult, to be honest). But you are running afoul of some pre established law by participating in these as a guidance has been issued. Since you aren't the one holding the money; they would be more likely to go after the ICO (this is about money, after all). Besides willfully purchasing an illegal, unregulated security, most of us are not accredited investors, which requires an AMAZING amount of income or wealth to qualify for. Only accredited investors would be allowed by the SEC to participate in something with this much risk. If you lie about being an accredited investor, or your region, to participate in one of these (they are starting to ask this now), you may forfeit your ability to seek damages/redress as you committed fraud.
Be careful. All of these arent securities, but for the ones that are, treat these as truly risky/volatile investments.
Agreed, I think that this issue needs to be discussed more. I feel that the majority of people who bypass region restrictions via VPN are ultimately worried about being about to retain their tokens. Also, the ability to eventually transfer to BitCoin and cash out to fiat. If the investor is scammed, that is their fault as I see it. Crypto investments are definitely a high risk/high reward venture. Not everyone will hit it big but wow what an exciting time to be in. There really isn't a firm US law blatantly restricting Americans from ICO participation. Only statements have been released regarding their position. I do however fully expect explicit laws within a couple of years. Until then, it's like the Crypto Wild West.