Basically, ICOs are 99% of the time classified as securities in the US. The main exception is if the token is non-transferrable or can be returned for what you paid for it. So, if you can't make money selling our trading it, it's not a security, which defeats the purpose of an ICO.
It sounds like who the SEC goes after is subjective, but at any point in time they could bring a suit against an ICO, even gimmick coins.