So I'll try to ELI5:

The US acts in a funny way because while we're one country, each state has the power to make its own laws. This is also why you hear about states like California and Massachusetts legalizing marijuana but it's still illegal in other places like Texas and Tennessee.

Same idea applies to Cryptocurrency. Some states, such as New York, have passed pretty strict laws about how to deal with it and report it, while other states like Kentucky are scratching their heads asking how to pronounce the word.

As a result, some new coin offerings are taking various approaches to deal with the US. Mostly, I've seen things like "You can buy unless you're in New York or any of the following states" but this is the first one I've seen that's just like "fuck you if you're American." Pretty bold statement for an American-made currency.

Anyway, this is the official statement from EOS. At the end of the day, no one can really stop you if you want to purchase in. I think the actual token sale is just the traditional "send us ETHEREUM and we will reserve some EOS for you." which can be done directly through wallets like "My Ethereum Wallet" but you just need to find out the address associated with the token sale.

That being said, I'm not saying you do it, and if you do, it's a risk that your investment might be confiscated by the US government if they find you.

Anyway, if you found this helpful I'd appreciate a follow!

Edit: it's not that owning the crypto would be any different, but crypto is still considered a currency by some states and there are regulations that crypto practitioners must follow, not just in the case of the buyer, but also in the case of the distributor. Maybe instead of conforming to all those rules EOS said "fuck it we'll just not sell in America."

Also, it could very well be they will allow it in the near future once the hype dies down from the rest of the world, so that they can pump up another hype train.