Hard to blame AMD for this...

Mantle was never intended for the Xbone or PS4, so it's pretty hard to get worked up about this supposed revelation.
You know darn well that Microsoft did everything they could to mandate DirectX as the backbone for the XboxOne graphics
Yup. DirectX is just a facet. Remember that D3D includes dependencies with DirectCompute, DirectXMath, XACT, D2D, and a raft of additional extensions

- MS would be stupid not to push their own proprietary API (which coincidentally also pushes their PC interests as well). Mantle being incompatible is most likely the result of a corporate power play. Quite simply, MS wouldn't have AMD hardware if it was tied to AMD software.

You might also consider that Mantle wont work with 99% of PC graphics. Intel IGP? Nvidia? AMD's own VLIW5 and VLIW4? No, No, and No. The exact reason why Mantle is a low-level API - it only deals with one particular architecture, and that won't change unless AMD change their stance on Mantle being proprietary to AMD...and if Intel and Nvidia are excluded then DirectX remains...and if DirectX remains then game devs have to include DX/OGL as well as Mantle....so a little extra work. I'm pretty sure that the devs wont be absorbing that cost- unless AMD are footing that particular bill.
Mantle is more a marketing tool..."OMFGWTF, look how much more frames per second the R9-290X gets in BF4!". Book it.

How it all plays out with the PS4 remains to be seen.

Not really. Sony already have two low-level API's and a wrapper. The chances that Sony would be happy to have AMD control application programming as well as hardware fit-out would be as miniscule as Microsoft relinquishing control.