I don't believe you're disallowed from participating in bounties simply because you are from the United States. Here's why:
ICOs are viewed as investments and traditional sales of equity. They're also viewed as securities. In the U.S., it's not legal for an entity to directly sell these investments to citizens unless they are accredited investors. That's why companies are allowed to sell ICOs to U.S. citizens if they show proof that they are accredited investors.
That said, alt and bitcoin bounties aren't part of this equation. Bounties are more like jobs or online mini-tasks where your reward is in tokens or bitcoin. That said, you're not investing money, but rather, you're receiving wages. Since that's the case, U.S. citizens can participate in most bitcoin and altcoin bounties.
If you go to the altcoin bounties section, you'll see you can participate in altcoin signature campaigns where you put the altcoin's signature as your signature. This is not an investment, because there is no sale of securities or equity. It's a job where you accept wages in tokens.
Bitcoin signature campaigns are the same way.
Additionally, there are bitcoin and altcoin campaigns where all you must do is retweet or like on facebook. These are also allowed for U.S. Citizens
