Marko,
PM can determine that the color space is sRGB by Exif tags and if you choose to embed an ICC profile we use the sRGB profile from your system, not Nikon's sRGB profile (which belongs to Nikon). I'm not sure of the exact differences between Nikon's sRGB profile and sRGB IEC 61966-2-1 but I suspect it may be a simplification of the sRGB standard. ICC profiles include a gamma coefficient, but since sRGB's gamma curve is formulated differently it includes a gamma curve
table rather than a simple gamma coefficient, and that is why the standard sRGB profile is so much larger in size than, say, an AdobeRGB or BruceRGB profile (the lookup table is bigger than a simple number).
If you really want Nikon's profile to be embedded then you will have to do it later in PM by selecting their profile as a custom profile. Since PM knows that it is sRGB by Exif tags, the "Default ICC profile if undetermined" logic doesn't apply (this is mostly for pre Exif 2.21 files that didn't shoot as sRGB - more than likely AdobeRGB but there wasn't always a safe way to tell for sure).
And technically, if Nikon is following Exif rules then the profile should really be the standard sRGB profile, right? I think they created their own profiles to avoid copyright issues.
Why don't you use AdobeRGB? Of course this same discussion could happen again since Nikon has their own version of AdobeRGB.
--dennis