they make the rules, but prohibition encourages growth of the "black market". So, it's up to them if they want to make their money by fighting crypto, or by working with it. It's not going to be entirely pretty either way. Crypto is here to stay. It will have value regardless of what the sec decides.
Mainly, it means that if the sec decides you are a security, you can't exchange your token on the american market without registering. This will add on a bunch of regulations most cryptos probably don't want to be a part of. First and foremost that you have to have detailed knowledge about everyone who invests with you, when offering a security in the united states.
However, I believe the end user would still be able to function, even if SEC names Steem a security. The problem is that it would be a lot more difficult for American investors, and for Americans who want to get their Steem exchanged.
related post: https://steemit.com/bitcoin/@mooncryption/breaking-news-sec-planning-creation-of-cyber-unit-with-ico-oversight