Because it is automatically turned on, its built into the engine. There is no way of turning it off. You get low fps because of OSX drivers.
It's not really built into the engine, rather into the OS X OpenGL framework since 10.4.7. So OS X Source isn't really multithreaded, as opposed to the Windows version. I don't even think that audio runs on a separate thread, since sound will stop in problematic situations even if the CPUs aren't saturated.
HL2_OSX will not use more than 140-150% of the CPU. I believe the application thread uses one full core (100%), and the openGL thread uses the remaining %, waiting on the main application thread (if it's not waiting on the GPU). Windows, Source will regularly go over 200%. What's more, OS X Source is significantly slower for the same amount of CPU usage (it's still slower if you disable multicore rendering on Windows). This might be due to inefficiencies in the CPU side of the openGL framework and drivers and to the direct3D translation layer they added. Overall, a Source game will be twice slower on OS X if performance is CPU-bound, that is, of you have a slow CPU compared to your GPU and/or low graphics settings.
Of course, if performance is GPU-bound (slow GPU/high settings), multicore support doesn't matter as much as drivers and openGL optimizations on the GPU side. These appear to be quite good on radeon HD cards, whose performances have improve at lot with the SL graphics update. At high video settings, OS X performance becomes similar to Windows. On the opposite, radeon X1XXX drivers appear to be quite bad. I believe Nvidia drivers could be better.
RBarris over at the Steam forum insinuated that they will be looking at "true" multicore support on the Mac sometimes in the future, but said nothing really concrete on that matter.
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