Each month, Themes Dojo (for example) gets 500+ referral websites sources to their themes or to demo content. That means thousands of unique visitors.
The majority comes from free download websites which buy (I would rather say steal) their themes and publish them for free download. The biggest question here is why Theme Forest doesn’t provide a free mandatory plugin which would activate the theme using their API in case the user is a buyer and has a license key.
The effect for authors is quite disastrous without a plugin like this – many of the themes are bought because users first downloaded them for free and in case they want them customized or have difficulties while using them. So, we are talking here about frustrated users which expect support that doesn’t necessarily mean support. It means advanced usage of the theme for custom purposes and cases, which in many cases is not covered by authors.
And the result is bad rating for authors’ themes for sale. Thought there are a lot of other users which are happy with using the theme without paying for them (and unable to provide good rating).
And yes, personally I think that this is a huge issue that has to be fixed somehow by the envato team by providing all the necessary tools to the authors to protect their content from being published on free downloads websites.