I'm not sure about this study, but there are cases where 'illegal' downloads have actually helped the producer of the product have better sales. When Radiohead came out Kid A the reviews considered the album "Just Awful"[1]. And I think that they never had any music videos or singles for the album, but a leak before the official release of the album gave people a chance to give the album a real shot. They liked it and they bought it. Kid A was Radioheads first release that became number one in the US and it also went platinum in the UK.
The reason it went number one is because it was a new album from Radiohead. Also people wouldn't have given it a real shot after the official release date?
Actually Kid A was the first album that made almost a genre change, which is why people considered it 'awful' in the first place: they were expecting the same old Radiohead and they got something completely different. So yes, had they released an album similar to the previous three, I would agree with you, but what they did was quiet risky and the free downloads allowed people to give it a chance.
[1] http://www.guardian.co.uk/friday_review/story/0,,371289,00.h...