Why do they power down ?

Steem has an anual expected inflation of more than 100 %, so it would be fucking stupid to not sell now.
This has nothing to do with not believing into this. Nobody would just hold their steem and happily watching its value dropping.

Is this a scam ?

Depends on the point of view I guess. But I did'nt see dan or ned asking anybody to invest into this, promising big returns if they do.
In fact most people just heard about simple posts making big profits on steemit. So they wanted this as well and signed up, started writing or maybe even invested into steem without even reading/knowing anything about it.
So this does'nt make this a scam but it shows, that a lot of people blindly invest into things they don't understand. This just shows: people love to be "scammed".

Overall the way, this platform is made might have a lot of things that do not seem to be fair for everyone. But this counts for the whole life, and I was thinking about how it could be done better without offering big chances for people to exploit it.
In fact I had to admit to myself, that a lot of their decisions just make sense for the future of this network, or better say: "For giving this network a chance to even have a future".

At the end they and a few others, secured their financial future with a good idea and the skills to get it running. Congratulations for this, you guys deserve it.
Even tho I can not say I would'nd be jealous and would prefer to have a piece of this myself. But I'm honest enough to myself, to admit that it was'nt me who had the idea, neither would have had the skills to do it.

In general humans love to complain and share their experience with the world. But there are not many who actually share their positive experiences, except with their closest friends maybe.
They brought a lot of people together, that would'nd have meet each other without this. A few people possibly made a bit money here, that they would'nt have got anywhere else. Some may deserve it - others don't - so is life !