BSD-like licenses seem to be the standard for userland graphics libraries. There's a lot of useful stuff in there (for example the shader compiler) that may be of general use elsewhere, and we wanted to make sure people could use it.
The 3-clause BSD is a compromise - we're happy for the stuff to be used but it's nice to get some credit :)
The 3-clause BSD is a compromise - we're happy for the stuff to be used but it's nice to get some credit :)