Originally there was a fear that people would gear away from spirit to get enough throughput from heal if it was too good, and just spam heal. So the spells were moved together in terms of HPM, Heal is still better HPM but not by ridiculous amounts.
In other words, heal and greater heal are very close in efficiency and you are not significantly penalized for choosing the wrong one.
I decided if this is true for non-priest healers using values from beta in tier 11 raid gear (premades). This is what I got:
Holy light is 25% more efficient than Divine light.
Heal is 10% more efficient than Greater heal (less when you account for Inner focus and train of thought).
Heal is 10% more efficient than Greater heal.
Nourish is 25% more efficient than Healing Touch.
Healing Wave is 43% (!!!) more efficient than Greater Healing Wave.
I'm not sure why the difference between priest heals is so small outside of it being intended, but the big difference for shaman is because Greater Healing Wave is extremely expensive when compared to Healing wave, much more than other healers.
This was done by comparing HPM between the big and efficient heals. This is a valid comparison between heals within a class, but not when you want to compare across class.