Epic gave their players right to use and redistribute their content, straight in the EULA.
(c) We just LOVE the idea of you using and distributing content or script from any prior Epic Games, Unreal franchise game in Unreal Tournament 2004 Mod. Therefore we grant you a license to use content from any prior Epic Games Unreal franchise game in your Unreal Tournament 2004 Mods. For the sake of clarity you will not gain any ownership whatsoever in any Epic content or script nor can you use any Epic content outside the scope of the rights granted here. Any attempt to do so will bring about the wrath of our attorneys.
The UDK EULA gives you right to use and redistribute included UT3 code for commercial purposes of your own game.
Also, US copyright law obliges them to pursue any copyright infringments. Any failures to do so may and will be used against them in court. So if you don't like that, go pester your politicians.