Why shouldn't Bump, Normal and Displacement maps be gamma corrected?
The fact that a photo is stored in gamma space doesn't have to do with PNG or JPEG, it has to do with the fact that it's a photo.

The camera detects a 50% intensity but is going to save it as a 187 instead of 127, because that is more efficient to keep the color information that matters to the human eye when you have only 256 different possible values. And so does Photoshop if you tell it so.

For normals or displacement maps, you don't have this need so there is no reason to use gamma space for them (using gamma for them would actually be a mistake, since it would affect how precision is distributed). Thus when you decode a normal texture or a displacement texture, you don't need to apply gamma.

If the normal map is encoded in a different space though, as can be the case for a G-Buffer for example, then you have to apply the corresponding correction.