When you want to contribute to an ico, they offer you the opportunity to contribute with Bitocoin, Ethereum, Litecoin and so many other coins, i dont know why you are saying that you can just put money with btc.
because it is false, and you can easily buy with ethereum because the coins will automaticaly be sent to your eth address if you invest money with this coin.
what is the reason that can only buy a lot of the ERC20 tokens with BTC and not ETH for example? Does Bitcoin actually holds a monopoly to enter small coins?
Just make a little research before posting this kind of questions, because everytime that you join an ico website, they have a big letter that says "Eth contributions"