whether they are treated as securities or not can only be answered ultimately by the SEC. Buuuuut, based on this recent speech by SEC official, steem is centralized enough to be in the running for consideration as a security.
https://www.sec.gov/news/speech/speech-hinman-061418
however, the sec is gonna go for the low hanging fruit first. Steemit Inc has a bunch of money, so the SEC is more likely to have a chat with them before just drawing charges.
I do, however, understand how they view crypto a lot better after reading the speech from Hinman.