Why are progressives so worried about this?

Why, it was just last year that Clinton herself assured all of us that she "did not email any classified material to anyone on my email." She went on to say: "I’m certainly well aware of the classification requirements and did not send classified material.” (http://www.nytimes.com/2015/03/12/us/politics/no-classified-emails-by-clinton-some-experts-are-skeptical.html)

So, what's the problem with Russia reading her emails?