This hot digital currency trend is minting millions, but US investors aren't allowed to play

For all the hype in the digital currency world about so-called initial coin offerings, U.S. investors often can't officially participate.

The reason, several analysts told CNBC, is companies' fear of being held accountable by the U.S. Securities and Exchange Commission in case buyers lose money in what can be a frenetic rush for something with unproven dollar value and sometimes prone to fraud.

"Everyone's afraid of the SEC. They can reach anyone in the world," said William Mougayar, author of "The Business Blockchain" and organizer of the Token Summit, a conference in New York this May about the digital coin business.

Not letting U.S. residents participate is a way for firms "to protect themselves because the SEC won't go after the customers, they'll go after the companies," Mougayar said.

The SEC declined to comment.

Before buying one of these new digital coins — based on the same blockchain technology as bitcoin (Exchange: BTC=-USS) — an investor typically must check a box that says he or she is not a U.S. citizen or green card holder. The coin sale websites also usually block U.S.-based computers based on their IP address.

Bitcoin has more than doubled in value this year, while the market value of all digital currencies has more than quadrupled this year, hovering around $80 billion, according to CoinMarketCap.

Screenshots of webpages for DAO.Casino (left) and EOS token sales

Source: DAO.Casino, EOS

An initial coin offering is a way for blockchain-based projects to raise funds, while allowing investors to own part of it through a digital network token. However, since most of the projects are still in very early stages, some worry about the rapid speed at which some projects are raising large amounts of money. About half of this year's roughly $1.2 billion in initial coin offerings was raised in the last month alone, according to a July report from financial research firm Autonomous NEXT.

Last Thursday, the Tezos blockchain project set a record for the coin sales by raising the equivalent of more than $200 million over the 13 days of its fundraiser. Switzerland-based Tezos said it did not specifically exclude investors from any country from participating.

U.S. residents holding bitcoin or ethereum could buy the Tezos tokens. But they couldn't participate in the fundraiser through its broker, Bitcoin Suisse.

"Due to the uncertainty with regard to crypto-assets and crypto-financial services from U.S. authorities, the IRS, and the FATCA [Foreign Account Tax Compliance Act] treaty, we are at this time unable to offer our services to U.S. clients," Bitcoin Suisse CEO Niklas Nikolajsen told CNBC in an emailed statement.