BTC is more like making an investment .An investment is something from which we expect to get huge returns in the future as compared to the normal savings we do in a bank.Yes we can also say that investing in btc is more like gambling but there are gambling options available in btc and in gambling we will either win btc or end up loosing what we have.where as making investment in btc is like waiting for the right time so that we can withdraw our investment in order to get profit out of the investment so investment on btc cannot be said as gambling where as doing gambling and trading is something which can be said so as it involve high risk.As far as my knowledge i dont think that btc gambling is illegal in the US as i never came across such news.