I think above ear height is a simple guideline. Just like 38% of room length for listener positioning, or 36 degrees for viewing angle.
If the speakers are above ear level, then you are no longer on the acoustic axis of the speaker, unless that speaker is angled towards you. If you angle the speaker, then only the person at that particular spot is on the acoustic angle.
I think the guidelines are there because, by far, most seat backs are too high and will block out the speakers. Also, there are blocking heads IF these multiple heads are on the same axis as the speaker.
Now, some people do prefer the freq response of a particular speaker when the tweeter is above ear level, even for the mains. But this is sort of a rarity.
Nutshell: above ear level for surrounds in most cases because it's simply practical in almost all situations. Ideal is ear level, however.