in the beginning, @ned got a lot of very cheap steem through tricking people by hiding the fact that he owned hidden accounts (ie ninja) and thus got lots of control of steem in ways that people didn't KNOW about. So they call that ninja mined because it was disguised and people weren't aware who owned those accounts.

Now they say that particular Steem is not valid, but they never blocked it before because they would've lost their witness votes. But since Jason Sun bought it and they are worried about him taking control and kicking them out, they decided to say NOW that the old steem that was ninja mined by Ned is not valid anymore. In other words, they don't like (ie fear) the new guy and so they are saying his Steem is no good and won't let him vote it.

My problem with it is that if they can do this to anyone, then they can do it to everyone. Frankly I'm very disappointed in many of my friends that are witnesses that support it. I'm sure they are just looking at it like they are "saving" the system (because they don't want the big bad Jason Sun to take over Steem), but in my opinion they are doing some serious damage to the value of the chain. If you can steal a guy's money so easily (Sun put in $10s of millions), then no serious investor will ever trust it won't happen to them.