Also negative numeric input emit light too.
Only changing the value with mouse is clamped.

Interesting! I'd say that this is technically not a bug...

Generally we do not clamp such negative values by design, for surfaces at least. For volume rendering it's not clear to me that this works in a useful way, probably best to clamp these values.

I agree, it's confusing when the volume emits light, although we don't have an emission closure. +1 for clamp.

The emission node and closure works for both surfaces and volumes, so emission is possible.

I realized it not emits light, but amplifies it.
Volume Scatter node does that too.
With black background the cube not visible, and the light only appears on the other object.

I just wanted to add my opinion here.
I understand that Cycles is supposed to be 'photo realistic'.
Nevertheless, I totally agree with smt that negative values are sometimes extremely interesting for artistic effects.
I have already used negative emission values to give me a 'black light' (increases shadows!).
So, yes, please don't eliminate this possibility.

If voxel textures arrive for cycles, that would be the perfect way to make a ghost for example. Or the smoke simulator for explosion, for fire could use perfectly that technically. (Becouse fire emit, and smoke absorb)Clouds on a nighscene. ect.

I tred recreate this scene with the emit node, and it just not worked out. This method gives a really great contrast!

I just played around a little bit with smoke from Brecht's demo file, and negative values. Here is the result. It uses only one sample.

I clamped the density input now to be >= 0.0.

If you really want to do these strange tricks you can still use a negative color or use OSL, but this should not be so easy to do by accident I think.