While the AHS can presumably add their own extra rules if they wish, they are the internationally recognized registration authority for the genus Hemerocallis and as such have to abide by the ICNCP (International Code of Nomenclature for Cultivated Plants).
Looking quickly through the ICNCP I can spot a couple of preclusions that might apply here. Also a name can't have more than 30 characters.
From the ICNCP:
You can't use a cultivar name that is so similar to another that it may cause confusion.
You shouldn't use a name that might give the impression the cultivar is related to or derived from another when it isn't.
This Stout rule may or may not relate to either of the above, but just to illustrate that the AHS has to abide by the ICNCP and therefore can't always do what the members would like.
