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.
Is the federal income tax the only tax that you are paying in us when you earn above $418,401?
No, it is just part of the tax bracket as a whole. If you are making $418,401+, you will be taxed at the highest rate (currently 39.6%). There are several different tiers of income that have their corresponding tax percentage. If you research "2017-2018 tax brackets", there will be a spreadsheet that details everything.