> Why in the world would they use Mesa?

Because it's an open source and full OpenGL implementation.

> The last time I check the Mesa API was not a licensed OpenGL implementation

Mesa doesn't have the license for the 'OpenGL' name. However:

"Mesa is a 3-D graphics library with an API which is very similar to that of OpenGL. To the extent that Mesa utilizes the OpenGL command syntax or state machine, it is being used with authorization from SGI. While Mesa is not a licensed OpenGL implementation, it is currently being tested with the OpenGL conformance tests. Despite these technical/legal terms, you may find Mesa to be a valid alternative to OpenGL. Most applications written for OpenGL can use Mesa instead without changing the source code."
http://www.opengl.org/documentation/implementations/#mesa

"There is an implementation of GL that is Open Source and it is called Mesa3D http://www.mesa3d.org
It doesn't have the license to call itself OpenGL, but it does follow the spec very well."
http://www.opengl.org/wiki/FAQ#Is_OpenGL_Open_Source.3F

(Btw, APIs are not implementations but specifications. Actual source codes are implementations.)

> rather it just used the OpenGL command syntax.

Yes, and its commands are supposed to do what the specification's commands with the same name do. That's exactly what's usually called an implementation of a specification (in this case of OpenGL 2.1 with several extensions from 3 and 4).

> I really meant a full, compatible, licensed implementation of OpenGL.

Like for instance? SGI's free but horribly outdated sample implementation of OpenGL 1.2? Or do you seriously consider the MorphOS Team to write its own full OpenGL (ES) implementation and obtain an 'OpenGL' name license from Khronos Group or from SGI?