Both Snowden and Greenwald have choosen to honour the truth above their own safety and security. Both of them are also acutely aware of the fact that standing shoulder-to-shoulder with Dotcom reflects poorly on their own integrity.
Maybe the USG leaked this email as a 'false flag' in order to make Greenwald look a fool when it gets proved as false, or it is true, and Greenwald and Snowden are right to back Dotcom in this particular argument, whether or not he is a crook or not.
A US movie company doing back-room deals (on behalf of the USG) with NZ politicians, in order to get someone extradited for alleged copyright infringement is just downright dirty and a waste of public money. It is corrupt corporatism at it's finest.
If it is true, I hope the shit hits the fan. However, as many others have pointed out. The fact that a politician is dirty is not an automatic guarantee that the voting public won't vote for him.