DXF is a drawing format, not machining format. The reason why many plasma controllers accept it is that a 2D drawing can be easily translated to torch movement. If you give a plasma machine a drawing of a 3x3" square, it will cut a 3x3" square.
LinuxCNC is a general purpose CNC controller. If you give it a drawing of a square, it would have no idea what to do with it. Cut a hole? Mill a pocket (how deep)? Engrave? It takes CAM software to translate a drawing to tool movements.
I guess a bending machine would use either G-code, or its own proprietary format.