KDE knows this, but they don't wish to be burdoned by the touchy legal situation admitting this problem would create for them, so they openly attack the GPL or outright ignore the problem. All that is required by a GPL'd application to be linked with Qt is a specific exemption for Qt from being considered part of the software. The KDE people seem to believe that those who object will find out their code is being used and make sure to tell them to stop using it. Forgiveness is easier to get than permission it seems. There are a few people out there genuinely hostile to KDE because of their clear belief they can do what they want because it's not like anyone is going to sue them for it.
Someone needs to talk about it.
Software's QuakeWorld source code released under the GPL last summer, is a prime example. First they tried to release source code for most of their QuakeWorld-based project, but leaving out a few key files necessary to compile or use it. Then they released a new version under the GPL's terms lacking the features they left code out of their last release for. Their excuses in all these cases are very similar to the excuses KDE has given people about their own legalities.