Valve cannot magically produce a homogeneous experience across all Linux distributions. They need some degree of consistency in the environment in order for users to have a reasonable experience. It isn't their job to prop up every two-bit distribution.
Well, just perusing through the files in my computer's home/.steam folder, it seems as though the binaries are packaged with all required dependencies, so as long as the Steam client works, the games should work on any Linux distro.
If the Steam client itself can be made to run on Slackware (I don't see why not, it runs on Arch just fine), the games will run as well. Steam installs all the dependencies, it's not dependant on distro-specific packaging...
Linux is Linux. As long as you have a display stack (ie. X) and graphics driver that provides OpenGL, Steam provides the rest of the dependencies. The Linux distro doesn't matter...
