Whether a single person passed along information they shouldn't have to the Clinton campaign, for unknown reasons (in the hopes of garnering favor? Because of opposition to Trump? We don't know) does not constitute "the mainstream media" actively colluding with her campaign. It constitutes a single person possibly colluding with her campaign. Even then, Brazile says it wasn't about the presidential debate (and that she didn't have access to questions for that)[1], but about a separate panel she was participating on:

A Democratic Party official suggested the question was actually prep for a panel Brazile was set to appear on, saying Brazile normally checked with both Democratic candidates for their positions. The day after the email was sent, Brazile appeared on ABC's "This Week" as a panelist.

Now, this isn't the best excuse, so I'm not sure I believe it, but I'm also not sure what I'm supposed to expect the Clinton campaign to have done. Loudly proclaim they got a town hall question beforehand when Brazile may have been misinterpreted, or wrong? Denounce her afterwards when they had already benefited from the knowledge, so people would vilify them anyway? If this was as bad as it looks, I'm still left concluding it was, until proven otherwise, and isolated incident, and one in which I think the campaign may have benefited, but ultimately didn't act in any way I wouldn't have expected them to (barring further information, such as them pumping her for more info). I also don't harbor any fantasies that the Trump campaign would have acted any differently.

Finally, I agree, it is journalistically horrifying. That's because journalism has it's own standards and ethics to consider. It was horrible for a journalist. But Clinton and her staff aren't journalists, they are politicians, and while it would be great if it wasn't true, we expect less from them. That's why journalism is so important and why it's protected under the first amendment, because it keeps politicians in check.

1: http://www.politico.com/story/2016/10/wikileaks-email-hack-c...