Because it sure looks that way. Sputnik News got the email wrong: they said it was Blumenthal who wrote it. Trump repeated the Sputnik error. No that the email was wrong, the Russian propaganda story was wrong about the email.
Correct, the email was not written by Blumenthal because he was just forwarding a Newsweek article. And correct, Sputnik came up with some really odd, stupid twisted way to present it.
But that's not the only thing that happened. Eichenwald in the end of his article was drawing the connection between Trump and Sputnik to suggest he was getting his news from Russian propaganda:
This false story was reported only by the Russian-controlled agency (a reference appeared in a Turkish publication, but it was nothing but a link to the Sputnik article). So how did Donald Trump end up advancing the same falsehood put out by Putin’s mouthpiece?
The problem was that there was a highly retweeted alt-right tweet several hours before that, and even a the_donald post about the tweet appeared a few hours before Sputnik. Eichenwald has now updated the article to include information about that, but that's fairly recent after Greenwald called him out on it. By that time it had already spread to CNN and other news orgs with the same incorrect claim.
What many? Do you mean Redditors? Me, I'm a lot more concerned with the tight connection between Russian propaganda and Trump. Maybe it was "just" that he gets his news from alt-right sources. Maybe he gets his news from public Russian propaganda sites. Or maybe he gets it directly from the Russian government.
Yes, Redditors. Eichenwald for a long period. People on Twitter. The 25,000 shares on facebook of a particular article in the screenshot by Greenwald. That is the point of Greenwald's article. People are willing to retweet, share, and spread articles that don't neccessarily have any factual basis just because it helps push the Russia connection.
Donald Trump getting his news from alt-right sources is still absurd for a candidate, but it's been known for a while just based on what he retweets. That's an entirely different story than jumping to the conclusion that he's getting his info from russia.