I think that Hidden condition of UI is not only written.
It works properly in 2.78a.

2.78a(ui.py)

device_type = context.user_preferences.system.compute_device_type
 if device_type in {'CUDA', 'OPENCL', 'NETWORK'}:
     layout.prop(cscene, "device")

I intentionally switched it from hiding the button to showing greyed out, it's not an issue in the code, though some might disagree with the design choice.

TBH I find the behavior of .active utterly confusing, I much prefer using .enabled. A button should either be disabled or enabled, not some state in-between.

If you changed it like that intentionally, I think that there is not the problem.
However, I can choose it as the gray out.
If it is selectable, do you not think that a chosen thing is used?

It's been a Blender UI behavior for a long time, when we did the initial button layout for 2.5 the convention was to only use .enabled if strictly necessary (when you could end up in an invalid state or so). I've always seen .active as more convenient than confusing, but I can see how it could seem strange.

The advantage is that you can change options in any order, and edit options that might matter but have no effect right now (due to animation, rigging, ..). It could be interpreted as trying to do the non-modal, non-blocking thing as much as possible. In this case for example it might be useful to enable GPU compute to send it to another computer for rendering, even if the computer you are working on can only do CPU rendering.

Not that I care particularly strongly about this, if the UI teams like to change it that's fine with me, just explaining the reasoning.

Thank you all for taking time to reply, and explain usage scenarios.

Since compute device selection now uses .active by default, would it help remove possible confusion for users if CPU always appears .enabled (since it always is), and GPU appears .active until enabled in user perseverances? See screen capture mockup.

@Brecht Van Lommel (brecht), Considering render farm usage, and GPU option appearing .active due to lack of local GPU compute device, it may help if the tool-tip is updated to reflect this usage scenario?

Tool-tip suggestions, also tried more condensed versions.

Current
Use GPU compute device for rendering, configured in the system tab in the user preferences

Proposed
Use GPU compute device for rendering, configured in the system tab of user preferences

Use GPU compute device for rendering, configured in user preferences (system tab)

Use GPGPU for rendering, configured in user preferences (system tab)

Proposed, helping explain various usage scenarios

Use GPU compute device for rendering, configured in user preferences (system tab).
Set also for GPU enabled render farms, even if local GPU compute device is unavailable

or

Use GPU compute device for rendering, configured in user preferences (system tab).
Set also if using a GPU enabled render farm, even if local GPU compute device is unavailable

I've now changed the greying out behavior to what you proposed, I agree that's more clear. I don't think the extra detail in the tooltip is needed, it's too specific an example.