I think there is an issue at stake here that people are ignoring. Trump's tweet really hurt Boeing's stock value. He now has the power to hurt companies by making tweets like this. This is government business he shouldn't be sharing that isn't set in stone. What's to stop him from telling his buddies at Goldman Sachs to buy Boeing stock low now, and that he isn't going to really cancel the contract later? Therefor the stock goes back up and they make lots of money? There are laws against doing stuff like this.
How is a "tweet" any different from him saying it at press conference?
It doesn't matter if it is in a tweet or a press conference. It will get picked up regardless. Do you know how many followers he has on Twitter? 16.9 million. His Tweets might as well be broadcast live on CNN, MSNBC, HLN, and Fox News simultaneously.
Even he knows how much influence his tweets have.
Donald J. Trump @realDonaldTrump Dec 5
If the press would cover me accurately & honorably, I would have far less reason to "tweet." Sadly, I don't know if that will ever happen!