Because the law believes that bits have colour:
http://ansuz.sooke.bc.ca/entry/23I'm not sure if this exact issue has been litigated, probably because the copyright holders would really rather not even discuss the issue, and it isn't the problem anyhow. A strict reading of the laws involved definitely show it's copyright infringement, but one could reasonably argue the appropriate damages for the act should be a flat $0. One can reasonably counterargue too, though. Something along the line of "Well, then they should have bought a second copy", though the problem with that is then you have to defend how when it is disadvantageous to the customer for it to be considered a strictly physical product, we consider it that way, but when it is disadvantageous to the customer to treat it as an abstract intellectual good, we treat it that way too. However, that's only a logical contradiction, not a legal one. This is probably why this has (to the best of my knowledge) never been litigated, even a victory of the case could set a precedent they wouldn't want. Best for them to just keep this out of court.