They have spent money or research developing the HARDWARE. Hardware is useless without software.
Spending money on hardware and not software is like designing a car without an engine.
Open source drivers allows them to provide a better "user experience" on Linux platforms, but more importantly it enables "peer review" of the driver which helps to indentify bugs, promotes trust, and provides a better "user experience" on some platforms (ie Linux).
So yes we are talking about open source, and we are talkign about financial benefits to AMD.